Dual-Band MIMO Antenna for n79 and sub-7 GHz Smartphone Applications

Abstract

The optimal design of Multiple-Input Multiple-Output (MIMO) antennas is significant for next-generation smartphone solutions with the appearance of further communication. This paper describes the design, simulation, and measurements of a ten-element MIMO antenna for n79 at 4.4–5.0 GHz, n102 at 5.925–6.425 GHz, and n104 at 6.425–7.125 GHz ranges that will be implemented in today’s smartphones. The proposed antenna system has a maximum dimension of 158 mm × 60 mm and will use the low-cost FR-4 as the dielectric material. The design procedure is carried out with a lot of care to ensure that it does not compromise the spatial characteristics and at the same time minimizes the mutual coupling between the nearby antenna dimensions. High isolation and low ECC values are simulated using capturing techniques on the antenna geometry and its element position for proper MIMO performance improvement. The constructed antenna system portrays good performances in the intended bands as evidenced by the reflection coefficient values, radiation patterns, and individual gains. The performance of the above proposed ten-element MIMO antenna system in terms of bandwidth efficiency, diversity gain, and channel capacity loss is discussed. It also satisfies the demands of current standards of wireless communications and provides future possibilities for higher data transmission rates and message reliability in next-generation smartphone applications. Here, the application of the FR-4 substrate is remarkable as it provides comparatively better performance with significantly easier manufacturing and less cost, and thus, the design has good possibilities to be commercially viable. This work can be understood to form part of ongoing research in enhancing the advancement of the smartphone’s antenna technology, which offers a scalable approach to the prospective spectrum altitude and the emergent surge in the data rates of mobile communication.

1. Introduction

5G New Radio (5G NR) can be described as a new radio access that has been developed exclusively for the 5G network. The term ‘air interface’ refers to technology that operates as a bridge between the handset and the base transceiver stations in cellular networks. Some of the new bands include n77 (3300–4200 MHz), n78 (3300–3800 MHz), and n79 (4400–5000 MHz), which have been assigned for 5GNR and are located below the sub-6 GHz range. To cater to the continuously increasing rates of throughputs and number of channels, the use of massive MIMO technology is vital [1,2]. It has also been seen that in the case of massive MIMO systems, the number of antenna array elements on both the Tx and Rx end improves the throughput dramatically. Therefore, the design of antennas for portable equipment and base stations is a critical issue in the air interface. As for bands below 6 GHz, sub-6 GHz 5G NR antenna designs at that time were concentrated at the bands of about 3.4–3.6 GHz, mainly including array antennae with operations at dual bands [3,4,5,6,7]. New developments have brought into the market MIMO antenna designs that can accommodate a broad bandwidth across all the 5G NR frequency bands bands n77/n78/n79 for use in 5G smartphone devices [8,9,10]. However, in designing these antennas, there is a tendency to rely on tedious and conventional full-wave electromagnetic simulations (EM).

The primary development trend for achieving the necessary spectral efficiency, bandwidth, and channel capacity is the deployment of fifth-generation (5G) networks. Considered the primary band of global interest for 5G communication, the C-band (3300–4200 and 4400–5000 MHz) is emerging as a pivotal frequency range for advancing 5G technology. It offers a balanced combination of coverage and capacity, making it cost-effective for widespread deployment. Since massive MIMO systems with many antennas may achieve enormous data speeds, one of the main areas of focus for 5G system developments [11] is when moving to the higher generations, higher frequency may cause short range and signal path losses, so to dynamically control the signal paths, researchers have used the reconfigurable intelligent surface (RIS) [12,13] with the existing infrastructure. Reduced isolation and decreased spectral efficiency are mostly caused by the designers’ inability to pack several antennas into the tiny area of cell phones.

Because of its superior spectral efficiency, a MIMO antenna with wider bandwidth or multiband operation and more separation between antenna parts is widely sought. Nevertheless, it is a difficult challenge to obtain improved spectral efficiency for 5G devices; it is proposed that a multiband antenna with tuning in all the frequencies sub-bands is best-suited. Several MIMO antennas with good isolation for 5G smartphones are recommended in the literature, including multiple antennas with low proximity [14,15], compact size antennas [16], and multiband MIMO antennas using monopole [17], dipole [18], and patch antennas [19,20]. However, the main drawback of such systems is that they possess higher local conversion coaxial [10,19] or deteriorated radiation characteristics [10,21] and involve higher complexity of design. In the work of [22], the design of a 10 × 10 dual-band MIMO antenna with a neutralizing line decoupling structure is demonstrated. However, its efficiency is poor at 45% and it provides isolation of only 12 dB.

As described in [23], an effective low-profile frequency reconfigurable MIMO antenna is designed for dual-band application using a varactor diode. This antenna was designed such that it can also control the output frequencies in the range of 1.7 GHz to 3.8 GHz for both bands at once. However, due to the nonlinear nature of a varactor diode, its fabrication process may become cumbersome and it also has a direct bearing on the radiation characteristics of an antenna. On the other hand, Ref. [24] presented a wide-band antenna for 5G within the sub-6 GHz bands using hexagonal split-ring resonators operating between 2.85 and 5.35 GHz with a 50% impedance bandwidth, but which suffers from an SRF split-ring resonator structure and native metal loss impacting the radiation efficiency of the antenna. These are some of the most commonly found methodologies for antenna designs especially for MIMO systems. But the search goes on for a multiband, independently tunable MIMO antenna with high radiation efficiency and port isolation, which is more demanding for 5G handsets utilizing the C-band.

Using the method of independent frequency control, a comprehensive parametric analysis was carried out to define assorted parameters. Moreover, a 10-element MIMO antenna array was developed and experimentally characterized to prove the design concept. The outcomes of this experiment show that the design mentioned in this paper provides outstanding results with high isolation greater than 17 dB. The antenna array also has peak efficiencies of 64% and 71%, and gains of 2.8 dBi and 6.2 dBi for the lower and upper operating frequencies, respectively. However, the novelty of this work is the design of open-ended scissor-shaped antennas with partial ground to achieve the n79, n102, and n104 frequency bands with connected ground for next generation applications. As well as these parameters, the MIMO antenna was also analyzed for diversity gain, and to determine the envelope correlation coefficient. These two parameters indicated that the design achieved near-perfect MIMO performance. The remainder of the article is organized as follows: Section 2 explains the steps involved in the design of the antenna and describes the geometry of the antenna in more detail. In Section 3, the s-parameters, radiation patterns, and MIMO parameters of the dual-band antenna are presented and analyzed with equally detailed consideration given to both the simulation and experimental results. Last of all, Section 4 of the article summarizes the entire study and the research findings.

2. Antenna Design

The flowchart in Figure 1 describes the methodology used for optimizing the MIMO antenna parameters. The approach includes a number of phases, such as the choice of operating frequency for the chosen antenna structure, the development of an optimized antenna design that fulfils the set objectives, and experiments with the improved manufactured prototype to confirm the efficiency and effectiveness of the new design. It starts with understanding how the ‘design parameters’, operating ‘frequency’, and the ‘performance’ of a MIMO antenna pair correlate with each other. Subsequently, the analysis also determines the best MIMO antenna pair to optimize its ten-element MIMO array, which is designed, simulated, and fabricated for testing. Data collection and design verification were performed using the ANSYS Electronics Desktop 2023 R2. The systematic approach to the MIMO antenna design employed by this methodology guarantees a one-stop solution for antenna parameter selection and ultimate validation on a test bed. The approximate X-shaped MIMO antenna is presented in Figure 2 with a fabricated prototype in Figure 3. The overall size of the MIMO antenna is 158 mm × 60 mm, while the dielectric material used for this purpose was Fr-4 with 1.6 mm thickness. The single antenna is fed by a 50 $\Omega$ feed line and the spacing between any two adjacent elements is approximately half wavelength at 5 GHz.

Parametric Analysis

Before moving on to the final results discussion, we are going to thoroughly discuss the final results achieved in six different steps. Following is the scenario in Figure 4a from which we obtained our desired results in Figure 4b:

Step 01:

Designing a Y-shaped antenna element with the full ground.

Step 02:

Designing an X-shaped antenna element with the full ground.

Step 03:

The slight shift of an X-shaped antenna to an open scissor-shaped antenna.

Step 04:

An open scissor lower section is filled while the ground is kept full.

Step 05:

In this step, the lower section filled open scissor using a partial ground shows some good results.

Step 06:

Finally, two slots were used to achieve the desired results.

3. Results and Discussion

In this section, we will discuss the simulated and measured results of a proposed antenna.

3.1. S-Parameters

To compare the simulated results with the measured ones, Figure 5 depicts the S-parameters of port 1 to port 5, that is, S11, S22, S33, S44, and S55. As seen in the plot, the measured values differ from the simulation results due to SMA connector soldering problems and the inaccuracies that are inevitable during the fabrication and measurement of electronic circuits. As illustrated in Figure 5, the achieved −6 dB impedance bandwidth does cover some of the desired bands, such as n79 (4400–5000 MHz) and the future upcoming band n102/n104 5.9–7.1 GHz. These deviations between the results obtained through measurement and simulation demonstrate the effect of practical issues that were experienced while designing the actual physical antenna.

The simulated and measured transmission curves (isolation) also reveal the working of the antenna and highlight that S21, S23, S43, and S54 are below −17 dB over the total operating bandwidth depicted in Figure 6. This low S21 value suggests that there is good isolation between the antenna ports that is required to avoid interference and ensure proper MIMO performance. These seemingly show the practicality of fabrication and measurement and indicate that the designed antenna is adequate to provide dual-band performance and high isolation. The obtained data provide evidence that the suggested configuration is appropriate for providing frequencies required in the contemporary 5G NR field.

3.2. Radiation Pattern

In order to have detailed information on the radiation behavior of the antenna, the far-field radiation patterns at 4.7 GHz and 6.7 GHz for the E-plane and H-plane are illustrated in Figure 7. When measuring these parameters and also when simulating the system, one of the ports is excited with 50 $\Omega$ impedance. The proposed antenna has near-stable radiation patterns of the omnidirectional kind in the H-plane at 4.7 GHz and 6.7 GHz. Consequently, the E-plane radiation pattern at GHz is bidirectional, and, therefore, a distinct and coherent pattern is easily displayed. But when the resonance is achieved at 4.7 GHz, the E-plane radiation patterns seem distorted. The deformation in the structure does not have any severe effects in terms of the overall radiation performance of the antenna. These observations suggest that the antenna can function with a favorable level of radiation across the examined frequency bands, as required for practical use. The steady H-plane patterns prove the usefulness of the antenna for applications that demand omnidirectional coverage, and the E-plane characteristics at various frequencies help in understanding the directional pattern of the antenna and behavior at the higher frequency. The 3D radiation pattern is presented in Figure 8.

3.3. MIMO Performance Parameters

In this section, we will discuss the MIMO performance matrices, the envelope correlation coefficient, the total active reflection coefficient, the mean effective gain, and the channel capacity loss.

3.3.1. Envelope Correlation Coefficient (ECC)

In [25,26], the theory underlying the ECC formula computation employed in Equation (1) is discussed. Using the following equation, which computes the envelope correlation coefficient (ECC) $\rho$, Figure 9 displays the ECC of the antennas 1 to 5 that are being presented. It is evident from Figure 9 that the simulated ECC value for all antennas in the frequency range of 4.4 GHz to 7.1 GHz is less than 0.002. The measured ECC is depicted in Figure 9. Equation (1) was used for calculation of the measured correlation from the measured reflection coefficients.

$${\rho }_{eij}=\frac{|S_{ii}^{*}{S}_{ij}+S_{ji}^{*}{S}_{jj}{|}^2}{(1-|S_{ii}{|}^2-|S_{ji}{|}^2\left)\right(1-|S_{jj}{|}^2-|S_{ij}{|}^2)}$$

(1)

3.3.2. Total Active Reflection Coefficient (TARC)

For the evaluation of MIMO performance, the TARC is an important parameter that considers all s-parameters. The TARC for the N number of antennas for the MIMO array can be defined according to [27].

$$\mathrm{TARC}=\frac{1}{\sqrt{N}}\sqrt{\sum _{i=1}^N{\left(\sum _{k=1}^N{S}_{ik}{e}^{j{\theta }_{k-1}}\right)}^2}$$

(2)

Figure 10 shows the simulated and measured MIMO parameters of the TARC. It is noticeable that in the relevant bands, both the simulated and observed values are less than −6 dB. The plot indicates that there is a modest shift in the measured response towards lower frequencies. This small discrepancy between the observed and simulated results could be caused by many factors, such as assembly mistakes and fabrication tolerance, but it still encompasses the entire bandwidth of the necessary bands of interest.

3.3.3. Mean Effective Gain (MEG)

The MEG (mean effective gain) is a significant measure to judge the MIMO diversity since it considers every antenna part separately in the multipath channel.

$${\mathrm{MEG}}_i=\frac{1}{2}\left[1-\sum _{j=1}^N{\left|S_{ij}\right|}^2\right]$$

(3)

This formula takes into consideration the power lost through the reflection and coupling effects of the antenna array from [28]. The MEG values are shown in Figure 11. The plot suggests that the simulated and measured MEG are almost the same and are less than −6 dB. The low MEG value is healthy for the system, as it indicates low losses and good gains in the multipath system. Further, a plot showing the ratio of MEG values from antenna 1 to antenna 2 is also presented. As seen in the findings above, both the ratios simulated and the ratios measured are substantially less than the 1 dB threshold and well below 3 dB. This low ratio provides strong support for the uniformity in the performance of the antenna elements, an aspect that is essential for good MIMO diversity. These MEG results show the effectiveness of the proposed MIMO antenna design to achieve a good diversity gain. The low MEG values and the small MEG ratio of the different antenna elements suggest that the antenna elements are harmoniously providing an efficient and reliable signal-capturing mechanism that forms the basis of effective MIMO communication.

3.3.4. Channel Capacity Loss (CCL)

Another important parameter of MIMO system performance is the channel capacity loss (CCL), which defines the loss of information during transmission through the correlated in-space antenna elements of the MIMO array.

$$\mathrm{CCL}=-{log}_{2}det\left(\psi \right)$$

(4)

The CCL simulated and measured data are presented in Figure 12 for all the simulated antenna pairs. The CCL values are computed for the same five antenna pairs as the ECC plot above. From the above analysis, it was found that the CCL values of all the antenna pairs are less than 0.4 bps/Hz, both in simulation and measurement. Consequently, the values of the upper bands are shifted to higher values. The simulated values spread over the lower bands are below 0.4 bps/Hz from [28], and the simulated values for the higher bands are below 0.5 bps/Hz.

In addition, the results that have been obtained are compared with those of the simulated results, showing that the actual MIMO antenna is more efficient. This implies that the antenna design does not lose much channel capacity, hence enabling adequate transmission and reception of information in the MIMO system.

The proposed MIMO antenna design is compared with recently published articles in

Table 1. Performance comparison of various state-of-the-art 5G antennas.

Ref.	Bandwidth (GHz)	Isolation (-dB)	ECC	Total Eff. (%)
Proposed	4.4–7.1 (−6 dB)	>17	<0.002	64–82
[10]	3.3–5.95 (−6 dB)	>15	<0.11	47–78
[29]	4.4–5.0 (−6 dB)	>11.5	<0.2	38–52
[30]	3.29–6.61 (−6 dB)	>16.6	<0.057	53–86
[31]	4.4–5.0 (−6 dB)	>16.5	<0.18	41–76
[32]	3.3–3.6 & 4.8–5.0 (−10 dB)	>9.9 & >12	<0.06	60 & 50

. The proposed MIMO design has higher isolation and ECC than the published ones. This paper presents a novel open-ended scissor-shaped antenna with partial and connected ground for n79/n102/n104 bands. This design can benefit high-frequency applications for the future such as 5G and IoT applications, enhancing inherited higher impedance matching, radiation efficiency, and the variety of connections for modern wireless systems.

4. Conclusions

This work presented a successful attempt at the design and performance analysis of a ten-element MIMO antenna system for the n79 (4.4–5.0 GHz) and 5.9–7.1 GHz frequency bands for smartphone integration. Using an FR-4 substrate was affordable yet effective and allowed us to construct the device with standard 158 mm × 60 mm dimensions. The design process helped solve the mutual coupling and spatial issues where the isolation level and envelope correlation coefficients (ECC) were high, which is desirable for MIMO operation. Simulation results obtained also validated that the proposed antenna system achieves optimum S11 levels, favorable radiation patterns, and reasonable gain levels in the desired bands of operation. Using criteria like bandwidth, efficiency, diversity gain, channel capacity, etc., it is concluded that the design is optimal for modern demands in wireless communications. The proposed ten-element MIMO antenna system not only guarantees improved data rate and reliability, but can be extended to future smartphone applications with future spectrum developments. Taking FR-4 as the dielectric material also demonstrated the feasibility of this design for manufacturing and its suitability for mass production without compromising greatly on performance. In general, this work contributes to the development of smartphone antennas and presents a potentially novel solution to meet the requirements of advanced, higher data rates and better communication quality for future mobile networks.