Design and SAR Analysis of Dual Band Antenna Array for 5G Cellular Communication Systems

Design and SAR Analysis of Dual Band Antenna Array for 5G Cellular Communication Systems

Danyal Ali

Department of Telecommunication Engineering University of Engineering and Technology Mardan, Pakistan

Abstract - This paper presents design and SAR analysis of a dual band 5G antenna array for future mobile communication. The proposed antenna operates in the Ka-band, at 28 GHz and 38 GHz. Roggers-5880 is used as a substrate having relative permittivity, thickness and loss tangent of 2.2, 0.254 mm and 0.0009 respectively in the design of the proposed dual band antenna. The main lobe gain of the single element dual band antenna is 7.71 dB at 28 GHz band and 7.73 dB at 38 GHz band respectively. As the gain requirement for 5G mobile communication system is 12 dB, thus an array technique is used to achieve the desired gain and produce bore side gain of 12.5 dB at 28 GHz band and 12.1 dB at 38 GHz band respectively. The dual band antennas matched with a VSWR < 1.16 in the two frequency bands. The designed antenna array is elliptically polarized at both bands of interest. The SAR analysis averaged over 10 g of tissue of the proposed dual band antenna array has been carried out on head of the human body giving a safer limit of 0.37w/kg at 28 GHz band and 1.34w/kg at 38 GHz band < 2w/kg according to European IEC Standard . All the simulations are carried out using CST MWS. The proposed antenna is a good candidate for applications in 5G wireless technology.

Keywords: Dual Band, Fifth Generation (5G), antenna array, ellipticall polarization, CST MWS

1) INTRODUCTION

From the early stage of wireless communication, higher bandwidth and data rate are the very basic requirements [1]. Until now 1G, 2G, 3G, and 4G wireless technologies for mobile communication has been completely evolved and deployed in almost all parts of the world [2]. The First-Generation wireless technology (1G) offers bandwidth up to 30 kHz, 2G up to 200 kHz, 3G up to 20 MHz and 4G up to 100 MHz [3]. According to studies, the data traffic exceeded the voice, increasing the need for a faster internet, both in relation to its cost and mainly to its performance. As with the widespread use of wireless devices and mobile services, there are still some issues that must be resolved, such as spectrum crises, low bandwidth and high energy consumption [4]. All these deficiencies forced to move towards Fifth Generation (5G) technology that has resolved this problem by providing very wide bandwidth up to 1GHz for wireless communication, very high data rate up to 1Gbps and low energy consumption [5]. Most probably 5G (Fifth Generation) technology will be implemented in 2020 [6].

A large proportion of millimeter wave spectrum (30-300 GHz) is unused and can be utilized for 5G [7]. A bandwidth of around 4 GHz is available at 38 GHz, while the frequency band 28 GHz offers a bandwidth of 1 GHz. Due to the availability of higher bandwidth the above-mentioned bands are of great interest in future research. Furthermore, 28 GHz and 38 GHz bands have minimum atmospheric attenuations [8].

The low-profile antennas are required to overcome the constraints such as size, weight, cost, performance, ease of installation, and compatibility. To meet these requirements, microstrip antennas are used. These antennas are low profile, conformable to planar and non-planar surfaces, simple, cheaper and compatible [9], thus microstrip patch antennas are a good choice for future 5G applications.

Nowadays, multiband antennas are essential for combining diverse communication standards in a compact single wireless system. In literature, different ways of designing multiband antennas have been proposed. For example, antenna with annular ring slots [10], elliptical slot [11], H-shaped slot [12] and rectangular slot [13].

The “FR4 PCB Grid Array Antenna for Millimeter-Wave 5G Mobile Communications” has been presented in [14]. However, the loss tangent of FR-4 is about 0.025. It is too lossy for millimeter wave antenna designs using traditional antenna structures such as printed patch antenna [15]. Thus RT/Duroid 5880 is the best candidate for millimeter wave antennas because of its low loss tangent about 0.0009 [16]. Furthermore, RT/duroid 5880 laminates are easily cut, sheared and machined to shape. They are resistant to all solvents and reagents, hot or cold, normally used in etching printed circuits or in plating edges and holes.

Being the most crucial and promising contraption in this time of remote correspondence, antenna need to ensure some safety of human body from its electromagnetic radiation, for that Specific Absorption Rate (SAR) analysis is conducted. SAR is the power absorbed by human body when expose to radio frequencies [17]. Usually this analysis is conducted on head [18]. According to European International Electro Technical Commission (IEC), SAR should be less than 2 w/kg per 10g of tissue [19].

This paper presents a dual band antenna for 5G mobile communication with L-shaped slots. It has been found that such slots have a significant impact on the performance of the antenna.

The rest of the paper is organized as follows: section 2, presents the design methodology of the proposed antennas. The results and discussions are presented in section 3. The SAR analysis has discussed in section 4 while section 5 summarizes the results and section 6 concludes the paper and gives future recommendations.

2) DESIGN

a) Dual band antenna

In this section the proposed dual band antenna design has been presented as shown in Figure 1. The geometry of the antenna consists of a 0.254 mm thicker low loss dielectric substrate material (RT/Duroid 5880) having relative permittivity and loss tangent of 2.2 and 0.0009 respectively. The substrate is backed by a finite ground plane having length L and width X is 9.3mmx9.3mm respectively. The total volume of the antenna is 9.3x9.3x0.254 mm3. The dimensions of the antenna (Figure-1) listed in Table-1 have been calculated from the well-known transmission line theory [20]:

The design formulae to calculate the width (W) and length (L) of the patch antenna are given below.

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Figure. 1. Geometry of the proposed single element dual band antenna (a) Front view

The summary of the designed parameters of the proposed antenna is presented in Table-1.

Table-1 Dimensions of the single element dual band antenna

b) Antenna arrays

As earlier mentioned that, gain require for 5G mobile communication is 12dB to overcome the path loss that occur by moving to the high frequencies of the electromagnetic spectrum. To achieve the required gain, array technique is employed in [21]. In figure-2 the geometry of 2x1, 4x1 and 2x2 array has been proposed. The spacing between the 1x4 array elements (Figure 2.b) has set to 0.25λ with an overall size of 135.128 mm3 while for 2x2 array elements (Figure 2.c), the spacing between the elements has set to 0.5λ with an overall size of 121.158 mm3 to avoid mutual coupling between elements. The width of the feed line for multiple elements can be calculated by [22]:

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(a)

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(b)

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(c)

Figure-2. Front view of (a) 1 x 2 array, (b) 1 x 4 and (c) 2 x 2

array

The summary of the designed parameters of the proposed antennas is presented in Table-2.

Table. 2 Dimensions of array antennas

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3) RESULTS AND DISCUSSION

a) Reflection Coefficient

This section presents detailed study of return loss, axial ratio, surface current, efficiency and gain of the proposed antenna. The proposed dual band antenna is designed and analyzed using Finite Integration Technique [23] (FIT) used in CST MWS to solve the electromagnetic wave equations.

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Figure-3. Return loss of dual band 5G antennas

The simulated results of the return loss |S11| for the proposed dual band 5G antenna are depicted in Fig. 3. It is observed that the antenna can operate at the dual frequency bands 28 and 38 GHz with |S11| less than -10 dB, which both are the candidate’s bands for the future 5G communications systems. The designed antenna works in dual band mode, 28 and 38GHz, with a return loss of -24dB and -28dB for single element, -24dB and -48dB for 1x2 array, -26dB and -23dB for 1x4 array while -38dB and -29.5dB for 2x2 array.

b) VSWR (Voltage Standing Wave Ratio)

The VSWR is the important factor that reflects the antenna performance. The designed antenna has voltage standing wave ratio (VSWR) less than 1.16 for both the two resonant frequencies which reveals that there’s a good adaptation between the feeding system and the proposed antenna.

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Figure-4. VSWR

The VSWR obtained at 28 GHz is 1.13, 1.13, 1.11 and 1.03 for single element, 1x2 array, 1x4 array and 2x2 array correspondingly, while at 38 GHz the VSWR is 1.08, 1.01, 1.14 and 1.07 for single element, 1x2 array, 1x4 array and 2x2 array respectively as shown in Figure. 4.

c) Axial Ratio

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Figure-5. Measured axial ratio of dual band 5G antenna at the bore sight

From the plots of axial ratios, it is analyzed that axial ratio is 20.86 dB, 24.645 dB, 16.2 dB, 6.65 dB at 28 GHz for single element, 1x2 array, 1x4 array and 2x2 array respectively, while for 38 GHz band, axial ratio is 11.4 dB, 16.6 dB, 14.54 dB and 11.88 dB for single element, 1x2 array, 1x4 array and 2x2 array respectively. The axial ratio values obtained, prove that the proposed dual band antenna is elliptically polarized at 28 GHz and 38 GHz. As if the axial ratio is greater than one on linear scale or greater than 3dB at the resonant frequency, the antenna is said to be elliptically polarized [24].

d) Radiation Pattern

I) Polar Plots (2D)

Gain pattern (in the H and E planes) of the designed dual band antenna at the two frequency bands is shown in figure. 6-7.

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(a) E plane

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(b) H plane

Figure-6. Gain pattern at 28 GHz band in (a) E plane (b) H plane

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(a) E plane

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(b) H plane

Figure-7. Gain pattern at 38 GHz band in (a) E plane (b) H plane

II) Far-field Plots (3D)

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(a) (b)

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(c) (d)

Figure. 8. Far field pattern at 28GHz (a) Single Element (b) 1x2 Array (c) 1x4 Array (d) 2x2 Array

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(a) (b)

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(c) (d)

Figure-9. Far field pattern at 38GHz (a) Single Element (b) 1x2 Array (c) 1x4 Array (d) 2x2 Array

The polar (2D) and far-field (3D) plots in Figure. 6-9 shows that the dual band antenna provides a peak gain of 7.71, 9.66, 12.5 and 12.3 dB at 28 GHz whereas giving peak gain of 7.9, 9.69, 12.1, 15 dB at 38 GHz for single element, 1x2 array, 1x4 array and 2x2 array respectively.

e) Surface Current Pattern

The surface current plots at 28 GHz and 38 GHz bands are shown in the figure. 8-9.

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(a)

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(b)

Figure-10. Surface current distribution at 28 GHz

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(c)

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(d)

Figure-11. Surface current distribution at 38 GHz

It is specifically to show the creation of current path to create resonance at 38 GHz. The 28 GHz radiation occurs from the edges of the patch and slot at the edges radiates at 38 GHz.

The efficiency of the designed dual band antenna is 92.6 %, 94 %, 92 % and 93 % at 28 GHz while 92.5 %, 97 %, 94 % and 91 % at 38 GHz for single element, 1x2 array, 1x4 array and 2x2 array respectively. Table. 3 summarize results of the proposed antenna.

4) SPECIFIC ABSORBTION RATE (SAR) ANALYSIS

Specific Absorption Rate analysis of the dual band (28 GHz and 38GHz) 2x2 antenna array on the human head has presented in this section.

Assessing the biological implications on the user may be one of the most important parameters for mm Wave 5G cellular devices. The level of electromagnetic absorption by the human body is strictly and universally regulated by governmental bodies under the specific absorption rate (SAR) guideline. Every cellular device must pass a regulated SAR test prior to certification.

SAR calculations have carried out over 10 gram of tissue volume (European standard). The safer limit for the standard is 2 W/kg for any 10 gram of tissue. The snapshots for SAR calculated on human head for 28 GHz and 38 GHz bands are shown in figure. 12-13.

Averaging Procedure

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1. Point of avg. SAR calculation
2. Search for 10 g cube (iteratively)
3. Integrate losses in cube

a) SAR of antenna array at 28 GHz

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(a) (b) (c)

Figure. 12. Specific absorption analysis of 5G 2x2 antenna array on human head at 28 GHz (a) Front view (b) Side view (c) Back view

b) SAR of antenna array at 38 GHz

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(a) (b) (c)

Figure. 13. Specific absorption analysis of 5G 2x2 antenna array on human head at 38 GHz (a) Side view (b) Zoomed in view (c) Zoomed out view

According to the results obtained, when 5G 2x2 antenna array is held on human head, it gives SAR of about 0.37234 W/kg at 28 GHz and 1.3031 W/kg at 38 GHz which is according to the European standard (must be less than 2 W/kg) in a safe zone and cannot damage any human tissue.

5) SUMMARY

The following (Table-3) summarizes the overall work.

Table-3 Summary of Results

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6) CONCLUSION

In this paper a dual band antenna array is designed for 5G mobile communication gives resonance at 28 GHz and 38 GHz bands. The gain of the single element is observed to be 7.71dB at 28 GHz and 7.9dB at 38 GHz, which are lower than the 5G standard, thus 2x2 and 1x4 arrays are made to achieve that particular gain require for 5G mobile communication. The gain achieved by the 2x2 array is 12.3dB at 28 GHz and 15dB at 38 GHz while gain achievement made by 1x4 array is 12.5dB at 28 GHz band and 12.8dB at 38 GHz band. It shows that gain limit set for 5G standard can be achieved by whether 2x2 array or 1x4 array. Then the effect of the radiation from the antenna on human health is analyzed through SAR analysis. The SAR obtained by mounting antenna array on human head is 0.37234W/kg at 28 GHz and 1.3031W/kg at 38 GHz. All the results are below the safer limit which is 2.0W/kg averaged over 10g of tissue according to the European standard. Hence radiations from an antenna will not damage any human tissue. The simulated result analysis gives good performances in terms of different antenna characteristics, since it provides high efficiencies, acceptable gains and good impedance matching at both frequencies of interest: 28 GHz and 38 GHz. Hence, the proposed structure demonstrates its promising potential for 5G applications.

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