Multi-Mode Smartphone Antenna Array for 5G Massive MIMO Applications

A multi-band antenna array is proposed for 5G massive MIMO systems. The presented antenna not only exhibits multi-band operation but also generates the polarization diversity characteristic which makes it suitable for multi-mode operation. Its configuration contains eight modified planar-inverted F antenna (PIFA) elements located at different corners of the smartphone mainboard. For ease integration and design facilitation, the antenna elements and ground plane are etched on the same layer. For $S_{11} \le -10 dB$, PIFA elements of the MIMO design operate at the frequency ranges of 2.5–2.7 GHz, 3.4–3.8 GHz, and 5.6–6 GHz covering the LTE 2600, 42/43, and 47 operation bands. Due to the placement of the antenna elements, the proposed design can support both vertical and horizontal polarizations. Fundamental characteristics of the proposed design are investigated. It offers good S-parameters, acceptable isolation, dual-polarized radiation coverage, and sufficient efficiency. In addition, the calculated TARC and ECC results of modified PIFAs are low over the operation bands.


I. INTRODUCTION
The fifth-generation (5G) cellular communication has received great attention from both academia and industry with a lot of reported efforts [1][2][3]. 5G will have significant improvements in transmission rate, latency, mobility and so on. MIMO technology with multiple antennas is probably the most promising technology to reach the transfer data rates of 5G cellular communications. It can enhance channel capacity and link reliability of system [4][5]. The greater number of antennas can make it more resistant to intentional jamming and interference.
2 × 2 MIMO systems are successfully employed for 4G mobile networks and the massive MIMO system with a large number of antenna elements is expected to be applied for 5G communications [6][7][8]. Recently, several MIMO antenna designs have been introduced for 5G cellular systems [9][10][11][12][13][14][15]. However, all these mobile-phone antenna designs either exhibit single-band operation frequency or use a few antenna elements with large sizes which could occupy a huge space of smartphone mainboard. We propose here a new design of MIMO smartphone antenna with eight elements which unlike the reported designs can cover multi-frequency bands simultaneously. Modified PIFA elements are employed at four coroners of the PCB to operate at three different frequencies which makes the design more suitable for Massive MIMO communications. The PIFA is a popular and relatively lowprofile antenna characterized by an omnidirectional radiation pattern [16][17][18]. Single-resonant, low-profile configuration of PIFAs suffer from impedance-bandwidth of multi-mode operation limitation [15].
The proposed MIMO PIFA system is designed to operate at three different bands including 2.6, 3.6, and 5.8 GHz of sub-6-GHz 5G cellular networks. The proposed MIMO system exhibits sufficient properties in terms of the fundamental antenna characteristics and could be used in future smartphones. The CST software is used to investigate the design properties [19]. The following sections present the antenna design details, single-element characteristics, and the performance of the proposed multi-band MIMO antenna.

II. DESIGN DETAILS OF THE PROPOSED DESIGN
The configuration of the designed MIMO smartphone antenna is illustrated in Fig. 1. As shown, it is composed of four PIFA pairs that have been deployed at different corners of the mainboard. The proposed antenna is designed on the FR-4 dielectric with a thickness of hx=1.6 mm. Each PIFA element is fed by a 50-Ohm discrete feeding technique extended from the ground plane to the antenna feedline. The parameter values of the design are listed in Table. I.  III. PERFORMANCE OF THE ANTENNA ELEMENT The configuration of the PIFA element is illustrated in Fig. 2 (a). Its structure is composed of an open-loop resonator with an L-shaped protruded strip. As shown, it has a compact size with a dimension of W×L. A 50 discrete feeding port is employed to excite the antenna. The main motive behind the modified PIFA is to obtain a compact antenna element which can support different frequencies and could be integrated with a smartphone mainboard while occupying small clearance. Figure 2 (b) illustrates the simulated reflection coefficient (S11) characteristic of the PIFA element. As shown, the antenna is operating at 2.6, 3.6, and 5.8 GHz with sufficient bandwidth and isolation characteristics. In order to justify the tri-band characteristic of the design, the simulated current densities of the modified PIFA element at different operation frequencies are illustrated in Fig. 3. It worth to mention that the maximum scaling for all figures is the same. At 2.6 GHz (first resonance), as can be seen, the L-shaped strip has a high current density with maximum current distribution. In addition, the current flow reverses on the interior edge of the surrounded open-loop. It is evident that the second resonance of the antenna S11 has been achieved using the open-loop resonator as it appears very active at 3.6 GHz. The third resonance can be considered as the second harmonic of the first resonance. As shown in Fig. 3 (c), the current distribution is almost equal around the L-shaped strip and the open-loop resonator. However, there is always some coupling between the employed square-ring slots, and the frequency response of the antenna is a result of these complex interactions [20][21][22]. The S11 characteristics of the modified PIFA antenna can be adjusted by changing the values for antenna parameters. Figure 4 illustrates the antenna S11 characteristic of varying design parameters including W2, W3, L4, and W. As can be observed, the antenna frequency response in all operation bands is very flexible to be tuned to lower or upper frequencies. Its impedance matching can be also affected by changing the parameter values  The fundamental radiation characteristics of the modified PIFA design including radiation efficiency (R.E.), total efficiency (T.E.) and maximum gain (M.G.) are represented in Fig. 5. As can be observed, the antenna provides sufficient efficiencies over the three operation band. In addition, the maximum gain of the design varies from 3 to 4.5 dBi. Figure 6 shows the S parameters (including Snn and Smn) of the designed smartphone antenna. As illustrated, the antenna exhibits good S parameters at three operation bands with acceptable mutual coupling less than -10 dB. The side view of the design radiation patterns for a singleelement radiator at different operation frequencies are shown in Fig. 7. Clearly, the antenna radiation elements provide high symmetric radiation patterns with covering the top/bottom sides of the mainboard and improving the radiation coverage [23][24][25].

IV. CHARACTERISTICS OF THE MAIN DESIGN
The 3D radiation patterns for the eight PIFAs of the main design are displayed in Fig. 8. It can be observed that the 8element MIMO antenna can offer sufficient gain vale for each radiator. As illustrated, the gain level of the design varies from 3 to more than 4 dBi. In addition, due to the placements of the PIFA element, four horizontally and vertically-polarized radiation patterns are achieved to improves the MIMO performance of the design [26][27][28]. Total active reflection coefficient (TARC) and envelope correlation coefficient (ECC) characteristics are two important parameters to be considered in MIMO antennas [29][30]. The ECC and TARC characteristics of antenna pairs can be extracted from the S-parameter results using the below formulas, respectively.
The ECC and TARC characteristics of the multi-mode smartphone MIMO antenna design are calculated and represented in Fig. 9. As evident from Fig. 9 (a), the calculated ECC results of PIFA pairs are very low entire the multioperation bands (less than 0.01). In addition, as illustrated in Fig. 9 (b), the TARC value of the design is less than -20 dB at different frequencies. V. CONCLUSION Design and characteristics of a new tri-band MIMO antenna with dual-polarization function are investigated in this paper. The configuration of the antenna contains eight modified PIFA elements deployed at four corners of the smartphone mainboard. The proposed design is operating at 2.6, 3.6, and 5.8 GHz for sub 6 GHz 5G mobile terminals. It offers good characteristics in terms of bandwidth, isolation, and radiation pattern. Due to tri-band and polarization diversity characteristics, the antenna can be considered for multi-mode applications.