Enhanced smartphone connectivity: dual-band MIMO antenna with high isolation and low ECC

The research presents an eight-element dual-band modified T-shaped slot antenna array optimized for seamless integration into contemporary smartphones, addressing the burgeoning demands of 5G communication within the sub-6 GHz spectrum. Operating across two critical frequency bands spanning 3.37-3.61 GHz and 4.9-5.1 GHz, this advanced antenna system boasts impressive efficiency and performance metrics. Symmetrically arranged on the ground plane, each antenna element measures 20 × 11.8 mm2 (equivalent to 0.233 λ×0.138λ), with a notable design feature being the inclusion of section Dx, strategically positioned between the ground-mounted antennas to enhance isolation among radiating elements by effectively managing surface currents. Fabrication results showcase isolation levels surpassing -14 dB, with an Envelope Correlation Coefficient (ECC) lower than 0.05 in the lower frequency band and 0.02 in the higher band, indicating robust performance in mitigating mutual coupling effects. The antenna array exhibits exceptional efficiency ranging from 48.5% to 63.7%, coupled with a commendable channel capacity of 38.8 bps/Hz and a gain of 3.9 dBi, highlighting its suitability for integration into smartphone devices and its potential to advance the next generation of mobile communication technologies.


Introduction
As wireless access and services spread globally, 5G technology has evolved from a theoretical idea to a reality.5G smart devices and base stations become increasingly necessary [1].Many antenna elements with broadband characteristics and high isolation may be required to provide 5G processing capabilities with enhanced multiplexing and spatial diversity within the target frequency bands.5G mobile networks offer an efficient solution for bandwidth constraints, low latency, improved data rates, higher connection density, and greater communication capacity [2,3].The 5G network system is poised to transmit data 1,000 times faster than fourthgeneration (4G) communication networks.Such exceptional data rates are facilitated by implementing multiple-input multiple-output (MIMO) technology [4].Higher data rates are achievable with MIMO due to reduced multi-path fading and lowered antenna coupling.To enhance diversity and multiplexing capabilities, the number of antenna elements must be increased to six or more.Recent developments in 5G cellular services have designated LTE bands 42 and 43 (3.5 GHz and 3.7 GHz) as preferred frequencies.Several antenna elements have been proposed as potential candidates for 5G mobile terminals [5][6][7][8][9][10].
In [5], a study presents six loop antenna elements measuring 150 mm × 75 mm ×0.8mm designed for upcoming 5G mobile phones.Four of these antennas are placed at the corners of the PCB, while two are positioned at the upper center.The design achieves isolation greater than 15dB, an ECC of less than 0.17, and a gain of 4 dBi.In [6], an eight-element MIMO antenna design is proposed featuring dual polarization.This design utilizes a ring square slot antenna excited by a rectangular microstrip feedline.To minimize mutual coupling between the two microstrip feeding ports, open-ended parasitic structures are incorporated on top of the ring slot, achieving isolation of up to 17 dB.In [7], researchers achieved isolation greater than 15dB by employing an Inverted L-shaped monopole combined with an L-shaped strip connected via the ground.Operating at 3.5GHz, this antenna configuration attained a gain of 4dBi and a calculated channel capacity of 38.1bps/Hz with an ECC below 0.1.Similarly, in [8], a 3.5 GHz antenna array comprising eight antennas was proposed.This array consists of four innovative parts, each comprising two mirrored antennas.Remarkably, isolation exceeding 10 dB was achieved without external decoupling elements, with a computed channel capacity of approximately 36 bps/Hz, validated through practical testing.Another study, [9], integrated a sixteen-antenna array into a smartphone for the 3.5GHz band.With a 20-dB SNR, the calculated channel capacity reached approximately 70 bps/Hz.Furthermore, in [10], the author introduced a 10-element array operating in LTE bands 42/43.At a 20 dB SNR, the measured channel capacity was 47 bps/Hz, with an isolation level of 10 dB.Despite achieving low ECC and covering the sub-6 GHz bands, there remains a need for further improvement in isolation levels.Effective decoupling of MIMO arrays is crucial for enhancing their diversity and multiplexing capabilities, particularly in the sub-6 GHz band where a 10-dB isolation level is deemed acceptable for 5G MIMO applications [11].Numerous studies have proposed various decoupling structures to enhance isolation, including the introduction of a neutralization line [12,13], addition of a protruding ground plane [14], inclusion of an etched slot [15], utilization of parasitic elements [16], implementation of pattern diversity [17], and integration of decoupling networks [18].However, employing external decoupling approaches adds complexity to the design process.Improving isolation without compromising total efficiency is achievable if external decoupling structures are avoided in MIMOantenna design.Nevertheless, the challenge of enhancing ergodic channel capacity persists, necessitating the development of antennas with satisfactory levels of isolation and efficiency.Various MIMO antenna decoupling methods, such as Defected Ground Structures (DGS), Electromagnetic Bandgap (EBG) meandered line resonators (MLR), artificial magnetic conductors (AMG), and orthogonal placement of radiating elements, have been employed to mitigate coupling [19,20].
This paper presents a novel dual-band eight-element array antenna for 5G mobile phone applications, covering frequencies from 3.37-3.61GHz and 4.9-5.1 GHz.Unlike previous designs that may involve complexities in various aspects, this antenna aims for simplicity in design, manufacturing, and MIMO characteristics.The achieved MIMO characteristics demonstrate excellent performance with an ECC below 0.05 and 0.02 across the two frequency bands, alongside an overall antenna efficiency ranging from 48.5% to 63.7% and a channel capacity of 38.8 bps/Hz.The SAR levels conducted are also found to be in safe limites.Remarkably, without the use of external decoupling structures, the antenna achieves isolation levels below 14 dB.Utilizing a low-cost FR-4 substrate and fed with an L-shaped microstrip feed-line, this proposed antenna offers a cost-effective and efficient solution for 5G communication in mobile devices.

Antenna design
Figure 1 shows the detailed dimensions of the designed antenna.The design comprises 8 elements labeled Ant. 1 through 8.A thin copper conductor sheet with a conductivity of 5.8 × 10 7 S/m is utilized on the ground, atop which lies an FR-4 substrate with a permittivity of 4.4, a loss tangent of 0.02, and a thickness of 0.8 mm. Figure 1(a) displays a single unit antenna element with a total size of 20 × 11.8 mm 2 (0.233 λ×0.138λ),where λ denotes the wavelength of free space at a center frequency of 3.5 GHz.The overall dimensions of the antenna are 150 mm×75 mm×0.8 mm 3 , as depicted in figure 1(b).The back-side view of the design is shown in figure 1(c).Part Dx serves as the decoupling structure between adjacent antennas, effectively reducing inter-element coupling.Its size is given by 6.5 × 1.2 mm 2 (0.076 λ×0.014 λ), denoted as d 1 and d 2 , respectively.An L-shaped feedline measuring 16 mm×0.8mm is employed to feed the ground-etched antennas.The detailed specifications of the array antenna element are provided in table 1.

Results and discussions
This section presented the results of the proposed dual-band eight-element array antenna, including antenna evolution, S-parameters, parametric analysis, radiation pattern, ECC, Channel capacity, MEG, antenna gain, overall efficiency, and diversity gain.

Antenna design evolution
The prototype design concept depicted in Figure 2 was implemented using CST version 2019, encompassing four evolution steps shown from case I to the proposed design.In the initial evolution step, case I, the T-etched slot lacked a middle slit (L c ), as shown in figure 1. Subsequently, in the following design stage, case II, an L c slit was introduced with a length of 12.6 mm and a thickness of 0.3 mm.In the third evolution, case III, a bent section W 2 , as depicted in figure 1(a), was incorporated while removing the inner bent section, as illustrated in figure 2. In the proposed design, elements from the three cases, I through III, were amalgamated to formulate the final proposed structure.
The scattering parameters plot of the evolving design is presented in figure 3, with letters E 1 to Ep used to denote the evolution cases from I to the proposed design accordingly.Specifically, E 1 corresponds to case I, E 2 to case II, E 3 to case III, and E p represents the proposed evaluation design.Analysis of the S11 plot reveals that E 1 exhibits resonance at 3.69 GHz, covering the frequency range of (3.57-3.8)GHz in the lower band, but fails to satisfy the -6dB impedance bandwidth in the higher band.For E 2 , resonance is observed at 3.57 GHz, with a frequency range of (3.45-3.69)GHz in the lower band and a center frequency of 5.06 GHz in the upper band.In the case of E 3 , a center frequency of 3.53 GHz is attained, covering the frequency range of (3.41-3.65)GHz in the lower band, and a center frequency of 5.2 GHz in the upper band.Finally, the suggested design E p achieves resonance at 3.5 GHz, covering the desired band 42 (3.37-3.61)GHz, and 5.0 GHz, covering the frequency range of (4.9-5.1)GHz in the upper band.The surface currents at both resonances are illustrated in figures 3(c) and (d).These figures depict induced currents at the edges of the resonating structures at both 3.5 and 5 GHz.The resulting isolation values for this evolution study are depicted in figure 3(b).The isolation level for case E1 was determined to be 13.8 dB, while for all other cases, the value was below -14 dB.It can be concluded that the incorporation of the horizontal slit (L c )  and the bent section W 2 shifts the resonance to a lower frequency, thereby achieving the targeted band of 3.5 GHz and 5 GHz in the upper and lower frequency bands with isolation levels lower than −14 dB.

Scattering parameters
Figure 4 showcases the fabricated design of the smartphone MIMO element featuring 8 SMA connectors via the ground plane to excite the individual elements, alongside the measurement setups in the anechoic chamber, employing VNA to obtain the scattering parameters.The MIMO antenna scattering parameter plots, based on simulated and measured data with optimized values under -6 dB impedance bandwidth covering the dual-band of (3.37-3.61)and (4.9-5.1)GHz, are depicted in figure 5(a).The simulated and measured results exhibit good agreement; any minor discrepancies may be attributed to soldering, fabrication, and the test environment.Figure 5(b) illustrates the plot when the decoupling structure Dx is not considered.
Based on the scattering parameter plot in figure 5(b), it can be concluded that the antenna covers the frequency range of (3.57-3.85)GHz in the low-frequency band, while in the higher band, the response does not meet the -6 dB criteria and cannot be quantified.Isolation of 13.2 dB was achieved in the lower band, indicating that the frequency does not adequately cover the desired operating band under −6 dB.This highlights the advantage of introducing the decoupling structure Dx, which enabled achieving dual-band frequency response and isolation below −14 dB.Due to the symmetrical nature of the antenna system, only some of the simulated and measured scattering parameter results are presented for brevity.In figure 6(a), the reflection coefficients for Ant1-4 are illustrated.All antennas in the dual-band exhibit a reflection coefficient magnitude greater than 20 dB.Due to the uniformity of the antennas, the reflection coefficient values appear to be virtually unchanged.
A single antenna's reflection coefficient is obtained by terminating the other antennas with an impedance of 50 ohms.Figure 6(b) illustrates the simulated isolation among the neighboring ports of the elements; isolation below -14 dB between any two ports was achieved over the two bands.Specifically, isolation of -14 dB was attained between antennas 1&2, 2&3, and 3&4, while for antennas 1&2, 6&4, and 4&8, an isolation well below -30 dB was achieved.The resulting variation is attributed to antennas placed nearby suffering from high coupling, mainly caused by near-field radiation.Figures 6(c) and (d) depict the antenna's total and radiation efficiency plots.In figure 7(a), the frequency response is directly influenced by the length L t of the T-slot, which can be adjusted to shift to the higher band in intervals of 0.5 mm. Figure 7(b) displays the corresponding transmission coefficient with all isolations well below −14 dB.The variation in the length of the slit (L c ) is illustrated in figure 8.As depicted in figure 8(a), the resonance frequency remains at 3.5 GHz, with only a shift in the frequency response magnitude as the length (L c ) increases from 10.8 to 12.6 mm with a 0.6 mm increment.The isolation level is also maintained below −14 dB, as shown in figure 8(b).In figure 9(a), as the value of W 1 changes from 3.6 mm to 4.8 mm with a 0.4 mm interval, the S 11 shifts towards the lower resonance with an increase in W 1 , achieving the desired resonance at 3.5 and 5.0 GHz with W 1 = 4 mm. Figure 9(b) illustrates that when W 1 is 3.6 mm, the isolation level is 14.2 dB, slightly better than the proposed one, but the resonance in the lower frequency band occurs at 3.7 GHz instead of the anticipated 3.5 GHz.Increasing the value to 4 mm, the isolation decreases to -14 dB.With a further increase to 4.4 mm, the isolation lowers to 13.8 dB; for 4.8 mm, it decreases to 13.39 dB.Thus, the port isolation decreases with an increase in the parameter W 1 .In figure 10(a), it is observed  that the lower the value of t 1 , the higher the frequency response shifts towards the upper-frequency band, and the better the reflection coefficient magnitude.
Isolation below -14dB was achieved consistently in all cases.Figure 11 shows how changing the length of the decoupling structure D x affects the antenna's performance.When the length decreases, the antenna resonates at higher frequencies, and when it increases, the resonance shifts to lower frequencies.The transmission coefficient, which measures how well signals pass through, remains constant at −14dB (Figure 11(b)).Similarly, figure 12 demonstrates that changing the width of the decoupling structure (d2) doesn't significantly affect the antenna's reflection coefficient.The resonance points slightly move towards higher frequencies, but the isolation remains steady at -14 dB across all cases (figure 12(b)).Finally, in figure 13, we see the measured results for the proposed antenna design.Figure 13(a) compares the simulated and measured scattering parameters, figure 13(b) displays the antenna's reflection coefficients, and figure 13(c) shows the isolation levels.

Radiation pattern
In figures 14-15, we present the dual-band antenna's measured and simulated 2D radiation patterns.As shown in figure 14   same and almost isotropic.For the same Ant.1-4 at 3.5 GHz with θ = 90 0 the antennas yielded maximum radiation along all the planes, with strong radiation at f = 60 0 , 240 0 , 300 0 , and 340 0 respectively.Similarly, Ant. 1 and 4 as can be seen in figure 14(b) and figure 15(d) at 5 GHz display a similar pattern at θ = 0 0 the only difference here is that strong radiation is observed at f = 210 0 , 270 0 and,300 0 for θ = 90 0 .For Ant 2 and 3 at 5 GHz, the radiation patterns are seen to be at 40, 210, 90, and 270 degrees, hence providing pattern diversity characteristics necessary for hand-held devices.The measured results are indicated with a dotted line and aligned with the simulated results.

MIMO diversity parameters
MIMO antenna designs are frequently characterized by envelope correlation coefficients (ECC), channel capacities, diversity gains, and mean effective gains (MEG), and thus become the most vital performance parameters for such kind of antenna design.The ECC is commonly used to calculate the diversity gain of multielement antenna systems, as it provides a correlation between the radiation pattern of two elements.For 5G MIMO systems, ECC levels below 0.5 demonstrate good diversity performance [21].The performance of the MIMO antenna declines when ECC is greater than 0.5, as there is a high correlation between antenna elements  and channel paths.The calculation of ECC is carried out either from the S-parameters [22] or by using far-field radiation patterns [23].In this work, the ECC is calculated using the letter-mentioned method as provided in equation ( 1).An ECC value of 0.05 and 0.02 in the lower and higher cut-off bands are obtained, as can be seen in figure 16(a) The antenna element diversity gain plots are shown in figure 16(b), The results show that all possible combinations achieve a gain over the operation band of more than 9.99 dB, and the DG is obtained from the ECC by using equation (2).A MEG indicates how well the system performs in a multipath environment.It is calculated using the equation presented in [21,22].Equation ( 2) is used to calculate the MEG from the measured results [24].To achieve good diversity performance, the difference of MEG should be below 3 dB.The difference in the calculated MEG between any radiating elements is lower than 1dB at the two center frequencies as provided in table 2.
Where B i (θ, f) and B j (θ, f) are the radiation pattern, while Ω is the solid angle while i th , and j th are the excited antennas.where the power received by vertically polarized antenna and horizontally polarized antenna is represented as P vpa and P hpa respectively.When calculating ECC and MEG, it is important to assume uniformity in incidents during calculation [24].The CC of the design is provided in figure 17.Based on the Kronecker channel model, the channel capacity is computed by averaging 100,000 Rayleigh-fading realizations.For the proposed 8 × 8 MIMO antenna, the transmitter is assumed to lack channel state information, and the receiver has a 20 dB SNR [25].Thus, from figure 17, it is obvious that the calculated channel capacity is 38 bps/Hz in the lower band and 38.8bps/Hz in the upper-frequency band, which in contrast to the 4 × 4 MIMO system is 2.59, and 2.64 times higher [26].Similarly, the channel capacity loss (CCL) is found to be less then 0.4 in both bands.Antenna total efficiency of 53%-65% over the operating band is obtained.The proposed design is compared with published literature in table 3.According to the the comparsion results as mentioned in table 3, the proposed MIMO antenna is a good candidate for 5G MIMO applications.SAR quantifies the rate at which energy from a radiating system is absorbed per unit mass of the human body tissue.Regulatory standards stipulate that SAR values should not exceed 1.6 W/kg for 1-gram tissue and 2 W/kg for 10-gram tissue to ensure safety.In our simulation setup, the mobile terminal was positioned 2 mm away from a realistic human head model obtained from a CST library, which accurately represents the electromagnetic properties of human tissue.Each element of the antenna array was driven with an input power of 25 mW, resulting in a total input power of 200 mW across all elements.Analysis of SAR was conducted to determine potential health risks associated with the device's operation.Furthermore, figure 18 showcases the SAR results for our proposed MIMO system.Our findings indicate that the MIMO system also maintains SAR levels within acceptable thresholds, reinforcing the safety of our proposed mobile terminal design.These results  underscore the importance of considering SAR compliance in the development of wireless communication systems, ensuring user safety without compromising performance or functionality.

Hand analysis
When designing antennas for mobile devices, it's important to consider the hand posture of the user.Nowadays, mobile phones are no longer just used for communication but also for other activities like gaming [23].Additionally, with the emergence of 5G technology, smartphones are expected to be used primarily for data rather than voice communication [27].This section will analyze and demonstrate how two common usage scenarios, single-hand mode (SHM) and double-hand mode (DHM), impact the MIMO performance parameters systems, including scattering parameters, efficiencies, and ECC.For this analysis, it is desired that the user's hands exhibit an electrical permittivity between 28 and 32, with an effective conductivity of 0.7 to 0.5 S/ per meter.To achieve this, a hand model is used with a conductivity of 0.8 S/m, permittivity of 29, and at center   It is evident that when in the SHM mode, both Ant.1 and Ant.4 experience a shift towards higher band frequencies for their lower and upper band center frequencies.However, the isolation level remains the same as the free space mode.The change in frequency is caused by the absorption of energy in the user's hand, as provided in figure 21(a).Additionally, figure 21 illustrates the port isolation parameters of the SHM and DHM, which in this scenario have a minimal impact regardless of the user's hand existence.A detailed analysis of the ECC for both modes of operation was conducted to shed light on how users' hands impact performance.The results revealed that the ECC is below 0.02 and 0.05 in the two bands for any pair of elements, as depicted in figures 22(e), and (f).The efficiency of the two modes is shown in figures 22, (a)-(d).Ant. 1 and 4 exhibit an efficiency of approximately 41%, and 52% in the lower and upper operating frequency range for SHM.On the other hand, antennas 2 and 3 have an efficiency of around 38% in the lower band and 49.5% in the higher frequency band.In the case of DHM, the efficiencies for the same set of antennas 1 and 4 are 35% and 46% in the  lower and upper bands, respectively.For Ant. 2 and 3 in the lower frequency band, the efficiency is 33%, while in the higher band, it is 43.4%.The system's efficiency has experienced a reduction, which is attributed to the dielectric loading causing a hindrance to the system's ability to operate optimally, leading to a decrease in efficiency.

Conclusions
An eight-element dual-band MIMO antenna system has been developed specifically for 5G and sub 6 GHz applications.The antenna offers an impressive impedance bandwidth of 240 MHz in the lower frequency band and 200 MHz in the upper band, meeting the -6 dB criteria.The system's key MIMO performance metrics, including ECC, efficiency, gain, and diversity gain, have been thoroughly analyzed and presented.The antenna exhibits excellent pattern and spatial diversity characteristics, resulting in channel capacity of 38.8bps/Hz.Efficiency ranges between 48.5% to 63.7%, with isolation surpassing 14 dB across the operational band.Additionally, the antenna's response to the user's hand has been examined, illustrating its behavior in single and dual-hand modes effectively.Close agreement between measured and simulated results has been observed, with  slight discrepancies attributed to testing conditions and manufacturing losses.The SAR results conducted yielded positive results.With the achieved outcomes, the proposed antenna system is good choice for future 5G smartphones.

Figure 3 .
Figure 3. Antenna evolution scattering plot and surface currents (a) reflection coefficient, (b) transmission coefficient, (c) Surface current at 3.5 GHz, and (d) Surface current at 5 GHz.

Figure 4 .
Figure 4. Antenna evolution scattering plot and surface currents (a) Top View, (b) back-side view, (c) VNA measurement and (d) Farfield Measurement setup.
(a), (c), and figure 15(a), and (c) at a low band frequency of 3.5 GHz, the radiation at θ = 0 0 are the

frequencies of 3 .
5 and 5.0 GHz.How the hand grasps, including finger position and whether the palm is placed on the right or left side, uniquely impacts antenna performance.To thoroughly understand and assess the different hand positions, the standard hand phantoms for both SHM and DHM, as shown in figure 19, are utilized.For SHM and DHM operation, the scattering parameters are provided in figures 20(a), and (b).

Table 2 .
MEG of the MIMO system.

Table 3 .
Comparison table of proposed MIMO Antenna.