Compact Four-Element Phased Antenna Array for 5G Applications

This paper presents a four-element compact phased Planar Inverted-E Antenna (PIEA) array for 6 GHz beamforming applications. For compact phased arrays, the mutual coupling is a severe performance degrading factor. Therefore, three mutual coupling reduction techniques are employed, which include (i) PIEA as an array element which is a modified version of Planar Inverted-F Antenna (PIFA) by inserting another shorting plate, (ii) slots in the ground plane and (iii) two slits in each etched ground slot. With these techniques, the mutual coupling is reduced below −19 dB in the operational bandwidth from 5.7 to 6.4 GHz. The compact design with an inter-corner spacing of <inline-formula> <tex-math notation="LaTeX">$0.013\lambda _{\mathrm {o}}$ </tex-math></inline-formula> and an inter-element spacing of <inline-formula> <tex-math notation="LaTeX">$0.3\lambda _{\mathrm {o}}$ </tex-math></inline-formula> is achieved. The peak gain obtained by this compact phased array is 8.36 dBi. This array can scan up to a maximum scanning angle of ±70°. A good general agreement is found between the measured and simulated results.


I. INTRODUCTION
5G represents a revolution in telecommunication, which will bring a paradigm shift in the way we communicate for the years to come [1]. It will provide a data rate up to 10 Gbps, fibre-like user experience, 1 ms over-the-air latency, user capacity up to several billion, and many more [2], [3]. For achieving this greater capacity, a large bandwidth is needed. Hence, 5G is looking beyond the presently used 3 GHz spectrum band. The spectrum of 3-30 GHz is referred to as microwave while 30-300 GHz is referred to as the millimetrewave (mm-wave) bands. In recent years, studies for the next-generation wireless communication systems have begun to shift from UHF frequencies (i.e. 300 MHz-3 GHz) to the new spectrum in the microwave range, including mmwave spectrums. Currently, the spectrum below 6 GHz is The associate editor coordinating the review of this manuscript and approving it for publication was Qammer Hussain Abbasi . not enough to meet future needs, which means that new spectrum allocations in the mm-wave frequency bands above 6 GHz are required to alleviate the current spectrum shortage. This is the primary driver in the development of fifth generation (5G) wireless communication systems. Hence, the microwave range above 6 GHz and mm-wave bands are the primary keys for resolving 5G challenges. Due to the increase in the number of subscribers and the need for high data-rate in wireless communication systems, the demands for an increase in channel capacity, mitigation of multipath fading, and co-channel interference are paramount for mobile and satellite communication systems. As the number of users increases, the co-channel interference also increases that leads to poor Quality of Service (QoS). This has triggered a lot of research effort in the area of phased arrays to tackle this problem and improve the quality of communication even in unfavourable conditions. On the other hand, electronic gadgets are becoming more and more compact.
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ Therefore, designing compact antennas and arrays with enhanced performance poses a challenge for the antenna design researchers [4]- [6]. Small cell base stations will require a compact beamforming antenna array. Thereby, the researchers have proposed different approaches for designing compact beamforming arrays such as a beamswitching array using Yagi-like parasitic elements and employing complex mutual compensation algorithms [7], [8].
The task of designing a compact beamforming array is also a challenge due to the mutual coupling being a severe performance degrading factor. The effects of mutual coupling intensify as the inter-element spacing is reduced. Planar Inverted-F Antenna (PIFA) has been a very popular candidate in portable wireless systems due to its appealing characteristics, such as low-profile, ease of fabrication and robustness etc. [9], [10]. Also, it does not require any matching network when connected to the 50 coaxial input [11]. It is also a suitable candidate for building compact arrays because it is comparatively less affected by another neighbouring PIFA [4], [12]- [15]. In the literature, few PIFA antennas and arrays are reported for 5G applications [15]- [20]. An 8-element phased array using open-slot PIFA antenna at mm-wave is designed for beamforming applications with an inter-element spacing of half a wavelength, reporting only simulated results [15]. In [16], a 4-element MIMO PIFA array is presented at 28 GHz. Three MIMO array models/arrangements are presented using space diversity where each element is located in a different arrangement with an inter-element spacing of half a wavelength [16]. A two-element MIMO antenna in which the elements are in anti-parallel arrangement facing each other is reported in [17] having an inter-element spacing of more than half a wavelength. Further, in [18], a three-element MIMO PIFA design is proposed with each element in a different direction/ arrangement, but no fabrication is reported. However, this MIMO antenna also has half a wavelength inter-element spacing. Another six-element MIMO PIFA was reported and designed at 28 GHz in [19]. In this design, the PIFA elements were arranged in different directions using space diversity with an inter-element spacing of half a wavelength [19].
In this paper, a design of compact phased array using Planar Inverted-E Antenna (PIEA) elements is presented having a resonant frequency of 6 GHz, by employing three mutual coupling reduction techniques, which are: dual-shorting pins, thus making it a PIEA instead of a PIFA, ground slots inbetween adjacent array elements and two slits in each of these slots with an inter-element spacing of 0.3λ o (where λ o = λ at 6 GHz = 50 mm).
The rest of the paper is organized as follows: Section 2 discusses the proposed array configuration. Section 3 presents the antenna array design approach using a parametric study. Section 4 compares the simulated and measured results, whereas Section 5 discusses the beam steering capabilities of the designed antenna array, and finally, Section 6 draws conclusions. Fig. 1 shows the configuration of a single dual-shorting PIFA element, thus becoming a PIEA. It consists of a ground plane with dimensions L g × W g . The PIEA top plate has dimensions of W × L, while the antenna height is h. The two shorting plates of width W s are located on each corner of the top edge of the antenna. Table 1 shows the optimised values of the design parameters of a single PIEA element.    2 shows the 3D view and back view of the compact antenna array having four PIEA elements. The inter-element and the inter-corner spacings are d and S e respectively. The ground slots are introduced in-between the adjacent array elements with a width equal to the inter-corner spacing of adjacent elements i.e. S e . Further, two slits named G slt1 and G slt2 are added in each of these slots at distances Y c and Y st , respectively from the upper edge of the array. The widths of slots G slt1 and G slt2 are W st and W c respectively. The dielectric material used is Roger RT/Duriod 5880 having a thickness t = 0.5 mm and a relative permittivity ε r = 2.2. Table 2 shows the optimised values of the design parameters of the compact PIEA array.

III. ARRAY DESIGN APPROACH USING PARAMETRIC STUDY
The compact array is developed by employing the design approach given as under: 1. Dual-shorting pin PIFA antenna, i.e. PIEA being more suitable regarding reducing mutual coupling explained in detail in the next section.  2. Introducing a ground slot between the adjacent array elements. 3. Introducing a slit in the ground slots at a distance Y c from the upper edge of the array. 4. Introducing another slit in the ground slots at a distance Y st from the upper edge of the array.

A. SINGLE ELEMENT ANTENNA DESIGN AND PARAMETRIC STUDY
The single-element PIEA is designed by applying parametric study approach to study the effects of antenna parameters on its performance. After observing the effects of all parameters, the design is optimised by using the Particle Swarm Optimisation (PSO) algorithm approach. The CST Microwave Studio (v. 2016) simulation software is used for this purpose. The width W of the top plate of PIEA is changed from 12 mm to 15 mm, while other parameters are held constant at W f = 2 mm, h = 1.9 mm, L = 7.5 mm, Wg = 14 mm and Lg = 22.5 mm. Figure 3 shows that the resonant frequency is inversely proportional to the width of the top plate as its value is decreased with an increase in the width. The width of nearly 14 mm is found to be suitable for the design. Further, the length L of the top plate of PIEA is varied while other parameters are held constant. Similar to the width of the top plate, it is found by the parametric study that the increase in the length of the top plate decreases the resonant frequency as being inversely proportional to each other. Next, the distance of the feeding plate from the shorting plate i.e. L b is changed from 2.5 mm to 5.5 mm while other parameters are held constant. Fig. 4 shows that the L b is an important parameter for obtaining the matching, and its value in the range of 3.5 to 4.5 mm can achieve the required resonant frequency. Further, the feeding plate width of the PIEA i.e W f is varied from 2 mm to 6 mm, while other parameters are held constant. The results in Fig. 5 suggest that the feed plate width affects the matching as well as the overall antenna bandwidth. The increase in feed width increases the bandwidth. Finally, with the values obtained from the comprehensive parametric study, the optimisation is performed by using the PSO algorithm. The optimised design values of single PIEA element are given in Table 1 as shown in the previous section.

B. ARRAY DESIGN AND PARAMETRIC STUDY
Like the single element PIEA, a parametric study is carried out to get an understanding of the behavior of different parameters and for obtaining the right size and positions of VOLUME 7, 2019 FIGURE 5. Parametric study for the width of the feeding plate W f . these different parameters for reducing the mutual coupling between array elements. In the parametric study, the same approach of changing one parameter at a time while others being held constant at the values provided in Table 2. When the position of G slit1 from the upper edge of the ground plane, i.e. Y c is varied from 14 mm to 20 mm, it is observed that it significantly affects the values of S 22 and S 12 but not affecting much to that of S 11, where S 11 and S 22 are reflection coefficients of PIEA elements 1 and 2 respectively and S 12 being the mutual coupling between these two elements. Therefore, the results related to S 22 and S 21 = S 12 are shown in Fig. 6. It is evident from the results shown that the value of Y c between 18 to 20 mm would show better performance in terms of S 22 and the reduced mutual coupling effect i.e. enhanced isolation between the two elements.
Similarly, Fig. 7 shows the effects of changing the position of G slit2 from the upper edge of the array i.e. Y st on mutual coupling S 21 . It is evident from the figure that the insertion of slit G slit2 and small variation in its position significantly affects the mutual coupling S 21 . And its appropriate position is required to achieve the maximum isolation between the two elements. An addition of slits at the position Y c and Y st jointly create a long slot in the ground plane, which blocks the coupling currents.
Finally, the inter-element spacing i.e. d is varied to obtain a compact design. In Fig. 8, the inter-element spacing d is varied from 15 mm to 25 mm with a step of 2 mm while all other parameters are unchanged. It is clear from the Figure that increasing the distance between elements increases the isolation but since we need a compact design, we need to have a minimum distance with the acceptable value of isolation. As the figure depicts that at an inter-element spacing of 0.3λ i.e. d = 15 mm, still provides good results with regards to isolation i.e. below −15 dB. Therefore, the separation d = 15 mm is suitable for the compact array design.
After iterative simulations, parametric studies and introducing mutual coupling reduction techniques described above, the inter-element spacing i.e d is achieved to be just  0.3λ = 15 mm whereas the inter-corner distance i.e. S e is just 0.65 mm = 0.013λ o while maintaining good isolation between antenna elements.

IV. RESULTS AND DISCUSSION
The simulated reflection coefficient S 11 in dB for single element PIEA is presented in Fig. 9. The results show that it  is a wideband design with a frequency ranging from 5.7 GHz to around 6.8 GHz for S 11 < −10 dB. It is found to have an adequate simulated gain of 4 dBi at 6 GHz. This PIEA antenna is compact and low-profile as the height of this antenna is just 1.8 mm. Secondly, it shows an inherent mutual coupling reduction capability when used as an array element which is shown in the compact phased array design. Fig. 10 shows the fabricated prototype of 4-element compact phased PIEA array. Figure 11 shows the simulated and measured s-parameters of the proposed compact PIEA array,  which confirms that the array is well matched and present a low mutual coupling between the ports, from 5.7 GHz to 6.4 GHz. The design achieves excellent performance with the use of the proposed methods, having isolation better than −19 dB in the operational bandwidth. The current distributions in Fig. 12 shows that shorting pins on both sides of the PIEA forms a built-in folded slot antenna in between,    which cause the coupling energy to radiate in the air and hence reduce the mutual coupling. Also, the slots between array elements, along with the two slits in each slot, block the coupling currents in the ground plane. The position of Y st decides the size of the folded slot antenna which affects the mutual coupling. Despite that, the mutual coupling is below −19 dB, which represents the effectiveness of applied techniques. Figure 13 shows the simulated 3D radiations patterns having the beam-steering capability at 20 • , 40 • , and 70 • .

V. BEAM STEERING CAPABILITY
The beam at 40 o is providing a 3-dB scanning range of 70 o , while the beam at 70 o is providing a scanning range up to 90 o . Figure 14 shows the measured 2D beam-steering capability at three angles by applying the adequate phase shifts between elements to have 20 o , 40 o , 70 o beams with the peak gain of 8.13 dBi, 6.8 dBi and 3.1 dBi respectively. The loss in gain is higher at far-angle due to higher cross-polarization as well as higher sidelobe levels at that angle. The results of active-S parameters in Fig. 15 show that the array is well-matched throughout the scanning range. Since only a few compact PIFA phased arrays exist in the literature, therefore, the comparison of the proposed work is made with DRA-type compact phased array and a PIFA array for mobile applications. Table 3 shows the comparison between previous works on the compact arrays and the proposed design presented in this paper. The design presented in [8] is compact as having an inter-element spacing of 0.32λ, but it has limited scanning range and lower bandwidth. The design shown in [7], is bigger as inter-element spacing is 0.4λ and also it has limited scanning range. Further, the design in [15] is bigger as inter-element spacing equals 0.5λ, and has a scanning range of up to 60 0 while no fabrication is done.
In comparison to these works, the proposed design is more compact with an inter-element spacing of just 0.3λ and has a scanning range of up to 70 o .

VI. CONCLUSION
In this paper, a compact phased PIEA array has been proposed, designed, fabricated and characterised for the upcoming 5G applications. Since the mutual coupling is a severe performance degrading factor in compact phased arrays, hence three mutual coupling reduction techniques are employed. With these mutual coupling reduction techniques, compact phased PIEA array with an inter-element spacing of 0.3λ o is achieved which can work from 5.7 GHz to 6.4 GHz, with a maximum beam scanning angle of ±70 o . However, the cross-polarization and sidelobe level are higher at far beams which will be addressed in the future work.