Due to the limited space and large mutual coupling levels, the design of sub-6 GHz massive Multi-Input Multi-Output (m-MIMO) smartphone antenna system attracts antennas’ designers and engineers worldwide. Therefore, this paper presents 18-element m-MIMO antenna system that covers the long-term evolution 42 (LTE42) frequency band (3.4–3.6 GHz) for the fifth generation (5G) applications in metallic frame smartphones. The proposed array system is etched on the long sides of a metal rim of the mobile chassis symmetrically, which is electrically connected to the system ground plane with zero ground clearance. A low-profile frame of height 7 mm (λ/12.3) is symmetrically placed below & above the ground system level. The two frame parts (above/ below the ground level) are utilized separately for the implantation of the antenna elements to achieve good space exploitation and perfect pattern diversity between elements. Orthogonal feeds are used to further increase the level of the isolation where each antenna element above the ground level is fed by a 50-ohm microstrip line, and each antenna element below the ground level is fed by a 50-ohm coplanar waveguide (CPW). The proposed antenna structure is a capacitive coupled-fed open-slot antenna with a small footprint area of 12 × 3.5 mm2 (λ/7.2 × λ/24.5, where λ represents the free space wavelength at 3.5 GHz). A small coupling L-shaped strip provides a capacitive coupling for the proposed 5G antenna structure. To establish the contribution of the proposed 18-element m-MIMO, the prototype of the proposed system was manufactured and successfully measured. Both measured and simulated results are shown to be in good agreement. This proves that the proposed antenna provides coverage for the LTE42 band (3.4–3.6 GHz) with acceptable isolation and efficiency. Moreover, the performance of the proposed m-MIMO were further verified via channel capacity calculation and the envelope correlation coefficient (ECC) measurement. Based on that, the proposed m-MIMO is shown to provide a desirable performance for all m-MIMO parameters while it owns the largest MIMO order in mobile terminals in the open literature.
After the huge success of using the MIMO technology in fourth-generation (4G) mobile communications, the fifth generation (5G) moves toward the deployment of massive MIMO (m-MIMO) technology. m-MIMO technology can further increase the channel capacity while mitigating the unfavorable effects of multi-path problems. Thus, it represents a key solution to meet the requirements of 5G technology [
Unlike previous generations, 5G suggests taking the advantage of deploying both microwave bands and millimetre wave bands. The former one is called frequency range 1 (FR1), which includes sub-6 GHz bands. The latter one is called frequency range 2 (FR2), which includes selected frequency bands above 24 GHz. In light of this, many works focused on producing massive MIMO antenna systems for sub-6 GHz smartphone applications [
The open literature includes several isolation techniques, external isolation structures like protruding ground branch [
Recently, various researches found that self-isolated MIMO antennas can alleviate the coupling problem. Such techniques achieve acceptable levels of total efficiency without increasing the overall size and system complexity [
The race of sub-6 GHz massive MIMO antenna design has been started since 2015. It started with 4 × 4 MIMO antenna systems [
Higher-order MIMO antenna systems like the 10 × 10 array have been also investigated in [
A 12-port antenna array was introduced for a dual-band 5G massive MIMO smartphone operation in [
In [
According to the aforementioned state-of-the-art, it is quite clear that the current research gaps in designing massive MIMO antenna system at 3.5 GHz smartphone 5G application are: (1) There are very few works having MIMO order above 12 × 12; only the designs reported in [
The most important contributions of this work can be summarized as follows: (1) The proposed antenna-element has a very small footprint area due to the feeding method that utilizes capacitive coupling to elements implanted on the metal rim of the smartphone. The proposed antenna structure is a coupled-fed open-slot element having many attractive characteristics; zero clearance area, low profile element (the total frame height is λ/12.3), and small footprint area (the longest dimension is λ/7). (2) the utilization of both pattern diversity (elements above and below the ground plane) and orthogonal feeds allows the installation of closely spaced elements up to nine elements on each long side of the metal rim. This allows obtaining a MIMO order of up to 18 × 18, which is not achieved yet for 5G 3.5 GHz smartphone applications.
Section 2 of the paper introduces the proposed 18 × 18 array system including its element geometry and array configuration. Section 3 presents an evolutionary process of the design and a parametric study of the effects of some vital design parameters. In Section 4, all the antennas’ and MIMO performance parameters results are introduced (scattering parameters, far-field 3D radiation patterns, total radiation efficiency, channel capacity, and envelope correlation coefficient). Then the state-of-the-art comparison is introduced in Section 5. Finally, a conclusion is attached in Section 6.
The overall geometry of the proposed 18-element MIMO antenna system is shown in
Due to the symmetric height of the metallic frame below & above the ground plane, the two spaces (below and above the ground plane level) are used for building two antenna groups. Pattern diversity is achieved from the existence of the system ground plane that inverts the radiation patterns in both elevation planes. Additionally, the elements of each antenna group are excited through a feeding scheme orthogonal to the feeding scheme of the other group. The etched elements on the upper portion form Group 1 (A1, A3, Ant 5, Ant 7, Ant 9 & Ant 10, Ant 12, Ant 14, Ant 16, and Ant 18) are fed by a 50-ohm microstrip feeding line each, while the etched elements on the lower portion form Group 2 (Ant 2, Ant 4, Ant 6, Ant 8 & Ant 11, Ant 13, Ant 15, and Ant 17) are fed by a 50-ohm coplanar waveguide (CPW) feeding line each. The microstrip feed line is excited through a vertically-placed 50 Ω SMA connector from the backside of the system PCB board (port Y). While in the CPW feeding method, a connector is connected horizontally (port X). For brevity, the models of connectors are hidden in
All the proposed antenna elements (Ant 1–Ant 18) are etched on the metallic frame along the two longer sides of the system PCB. Each side has nine elements identical to the arrangement of other side’s elements and distributed as follows: five elements are etched on the upper half of the frame (Group 1), while the remaining four elements are installed on the lower half of the frame (Group 2). The elements of these two groups are distributed in an alternate fashion.
This section aims to provide a detailed explanation of the design evolution process, and the effects of vital design parameters of the proposed antenna element.
As shown in
REF1 represents an Inverted-L-shaped feeding strip that excites a small rectangular notch that is created on the metallic frame as shown in
For an extra explanation of the working mechanism of the presented antenna, the effects of some vital structure parameters are investigated in this section. As it was clearly shown that both the gap and the stub have the main contribution to the operation of the proposed antenna-element structure, their locations and dimensions were taken into consideration very carefully to tune the results. These parameters are as follows: the position of the gap, the gap length (Lg) and finally, the stub length (Ls). It is worth noting that the other design parameters are all kept constant during the study of the effect of each aforementioned dimension separately. As the effect of these dimensions is mainly on the reflection coefficient, only the reflection coefficient results are presented. Ant 5 is chosen for this study due to the symmetry.
The gap length (Lg) effect is attached in
The proposed 18-element m-MIMO antenna has been fabricated to validate simulation results. The manufactured prototype is shown in
The simulated results of the proposed massive MIMO antenna are achieved using CST Microwave Studio [
The simulated and the measured isolation parameters among antennas of the same group (co-group elements) are shown on
In order to show the achieved pattern diversity among adjacent elements (Group 1 and Group 2), the simulated 3D radiation patterns at the center frequency (3.5 GHz) are shown in
In order to validate the simulation results, the normalized far-field radiation patterns in principal planes were measured inside an anechoic chamber as shown in
The envelope correlation coefficient (ECC) and the ergodic channel capacity represent the most important MIMO performance parameters. Therefore, this section evaluates them in free space.
The ECC represents a measure of how much received diversity signals, by two MIMO antenna elements, are different. Thus, ECC needs to be as low as possible to ensure excellent diversity performance. Practically, ECC values of below 0.5 are acceptable in recent 4G and 5G MIMO systems [
The ergodic channel capacity (CC) is evaluated using
The channel matric H is evaluated using
The received and transmit antenna ECC matrices are calculated as follows:
The calculated ergodic channel capacity of the 18 × 18 MIMO antenna system, which is described above, is calculated from the collected measured results by averaging 20,000 independent identically distributed fading channel realization with SNR equals 20 dB [
This section aims to introduce and summarize the values of the proposed design in comparison with other antenna solutions for Smartphone sub-6 GHz 5G massive MIMO antennas.
Ref | Bandwidth (GHz), dB | MIMO order | Ground clear (mm) | Elements |
Channel capacity (bps/Hz) | Isolation (dB) | Total efficiency (%) |
---|---|---|---|---|---|---|---|
[ |
3.3–3.6, −6 | 8 | 4.5 | 17 | 35 | >15 | >40 |
[ |
3.4–3.8, −6 |
10 | 3 | 18 | 43.3 | >11 | >55 |
[ |
3.4–3.8, −6 | 10 | 3 | 20 | 47 | >10 | >42 |
[ |
3.4–3.6, −6 | 8 | 2 | 19.4 | 37.9 | >10 | >45 |
[ |
3.4–3.6, −6 | 16 | 3 | 20 | 70 | >10 | >30 |
[ |
2.5–2.6, −10 | 8 | 4 | 13.6 | 40 | >12 | >48 |
[ |
3.4–3.6, −10 | 8 | 0 | 11.2 | 35 | >20 | >60 |
[ |
3.4–3.6, −10 | 8 | 0 | 20.8 | 34 | >19 | >60 |
[ |
3.4–3.6, −10 | 8 | 0 | 15 | 35 | >10 | >40 |
[ |
3.4–3.6, −10 | 4 | 1 | 1.2 | 18.3 | >17 | >50 |
[ |
3.4–3.6, −10 | 4 | 1 | 1.2 | 19 | >17 | >55 |
[ |
3.4–3.6, −10 | 8 | 3 | 17 | 40 | >10 | >56 |
[ |
3.4–3.8, −6 |
10 | 3 | 30.2 | 51.4 | >10 | >42 |
An 18-antennas m-MIMO array operating in the LTE42 band has been presented for a 5G metal-rimmed smartphone. The proposed system has the highest MIMO order over the open literature. The MIMO array utilizes zero ground plane clearance between the metal rim and the system PCB board. One more added and selling feature of the proposed design, is a self-isolated MIMO antenna design as both patterns diversity and orthogonal feeds are used. The proposed antenna-element structure is made of a small footprint area (the longest dimension is λ/7) and a low profile (each element height is λ/24.5). It is an open-end slot excited through a capacitive L-shaped strip. Additionally, the main merit is that it’s a system that introduces the array with the largest number of antennas in m-MIMO technology for the smartphone. The results show that the LTE42 band is covered under the −6 dB criterion of impedance matching, isolation level better than 10 dB without using external decoupling circuitry, and total efficiencies are about 45%–68%. furthermore, a good MIMO performance is proven by the results of the channel capacity and ECC. Consequently, the results prove that the proposed array system is a promising solution for large channel capacity 5G massive MIMO array in metallic frame smartphones.