Highly-Integrated Millimeter-Wave Wideband Slot-Type Array ...

Highly-Integrated Millimeter-Wave Wideband Slot-Type Array Antenna for 5G Mobile Phones

Wei-Yu Li1, Wei Chung2 Information and Communications Research Laboratories

ITRI, Hsinchu 31040, Taiwan [email protected], [email protected]

Kin-Lu Wong Department of Electrical Engineering

National Sun Yat-sen Univ., Kaohsiung 80424, Taiwan [email protected]

Abstract—A highly-integrated wideband slot-type array antenna formed using parallel-connected dual open-slots as the array element for 5G mobile phones is presented. The proposed wideband millimeter-wave array antenna can achieve 5.6 GHz bandwidth (-10 dB S11 definition) from 23.9~29.5 GHz to cover the whole 26 GHz and 28 GHz bands (n258 and n257 bands) of 3GPP 5G NR specification. A 4-element linear array antenna prototype is implemented by using simple printed circuit board (PCB) process for experimental study. The 4-element linear array antenna occupies a small layout area of 21.3×1.64 mm2 at the short side edge of a PCB. Results show that wide-coverage fan-shape beamforming patterns and wide beam scanning coverage about 120 degrees can be achieved.

Keywords—5G antennas; millimeter-wave antennas; wide-band array antennas; mobile phone antennas; slot-type array antennas

I. INTRODUCTION

In order to solve the global bandwidth shortage in current wireless cellular networks, one promising solution for the fifth generation (5G) communication system is expected to utilize millimeter-wave (mmWave) bands [1-7]. For mmWave communications, the phased array antennas which can achieve high-gain, beamforming, and beam scanning are expected to be widely used to overcome the much higher propagation loss compared to the 4G communication system using sub-6 GHz bands [2-8].

For possible 5G NR spectrum allocations at lower mmWave frequencies, a 26 GHz band (n258 band, 24.25~27.5 GHz) and a 28 GHz band (n257 band, 26.5~29.5 GHz) are defined in Specification TS 38.104 of 3GPP release 15.1 [1]. The 26 GHz and 28 GHz bands are popular for many countries to develop mmWave field test due to their relatively lower path losses compared to another higher 39 GHz band (band 260, 37~40 GHz) [1]. Therefore, many 28 GHz band array antenna designs for 5G mobile phones have been studied in recent years [2-7].

However, different countries may plan to use different frequency ranges of n257 and n258 bands for lower mmWave band communications. For examples, Japan may use 27.5~29.5 GHz band, Korea may choose 26.5~29.5 GHz band, China may choose 24.75~27.5 GHz band, and US may use 24.25~24.45, 24.75~25.25 and 27.5~28.35 GHz bands. Nevertheless, the mmWave array antennas published in the prior studies generally cannot cover the whole n257 and n258 bands for fulfilling different country applications [2-7] .

For this motivation, a novel PCB highly-integrated mmWave wideband slot-type array antenna covering the whole n257 and n258 bands and achieving wide beam scanning coverage for 5G mobile phones is presented.

II. PROPOSED WIDEBAND ARRAY ANTENNA

Figure 1 shows geometry and experimental photos of the fabricated wideband 4-element slot-type linear array antenna. The array antenna is implemented at the short edge of a 0.2 mm thick rectangular PCB (Rogers 4003C) treated as the system circuit board of a mobile phone. In order to achieve wideband operation, the array antenna is formed using parallel-connected dual open-slots as the array element. The center spacing between adjacent array elements is chosen to be about 0.5λ of 26.5 GHz (5.36 mm shown in Fig. 1) for suppressing grating lobe effects during beam scanning [9].

Figure 1. Geometry and experimental photos of the proposed mmWave wideband 4-element slot-type array antenna.

Figure 2. (a) Simulated E-filed vector strength of the two open slots at 26.5 GHz; (b) Simulated and measured S parameters for the adjacent port 1 and port 2 of the proposed array antenna.

The proposed array element mainly consists of two spaced open slots formed on the backside ground plane of the PCB and a coupling strip printed on the front side of the PCB. The length of each open slot is about 0.25λe (free-space wavelength divided by square root of 3.55) of 26.5 GHz considering the effect of dielectric constant 3.55 of the PCB. The two open slots are spaced apart by about 0.4λ of 26.5 GHz. The function of the coupling strip is for exciting the two spaced open slots. There is a printed microstrip line connecting to the coupling strip to excite the two open slots.

Figure 2(a) shows the E-field vector strength of the two open slots for different center-fed case (Reference array element) and off-center-fed case (Proposed array element) at 26.5 GHz frequency. Considering the dielectric effects of the PCB (Dk=3.55), the off-center-fed case of the proposed array element is designed for effectively causing about 0.5λe (180-degree) phase delay between the two spaced open slots. Therefore, it can be seen that the off-center excitation can induce coherent E-field vectors at the two open slots for achieving directional and broadside radiation pattern toward +z direction [9].

However, the reference array element with the center-fed excitation will induce opposite E-field vectors at the two open slots which will lead to radiation cancellation in far field region. The measured radiation efficiencies and peak realized gains of the proposed array element in the desired frequency bands are respectively about 70%~86% and 2.5~3.4 dBi. More results of the proposed array element will be presented and discussed in the presentation.

III. RESULTS AND DISCUSSION

As shown in Figure 1, the proposed 4-element array antenna occupies a small layout area of 21.3×1.64 mm2. For performance testing, the microstrip lines are used to feed the 4 array elements and connect at points a, b, c, and d to 50-Ohm mini SMP connectors (ports 1, 2, 3, 4) soldered on the backside ground plane of the PCB. Figure 2(b) shows the simulated and measured S parameters for adjacent port 1 and port 2 (as shown in Figure 1) of the 4-element array antenna. It can be seen that a wide operating bandwidth of total 5.6 GHz (-10 dB S11 definition) can be generated to cover the whole 26 GHz and 28 GHz bands (n258 and n257 bands) of 3GPP 5G NR specification [1] to fulfill different 5G NR spectrum allocations of different countries at lower mmWave frequencies. Besides, the mutual coupling of the measured adjacent feed ports (port 1 and port 2) is less than -14 dB in the desired n258 and n257 bands which is sufficient to overcome the performance degradation caused by beam steering.

Figure 3. (a) Simulated 3-D radiation patterns for additive phase shift p and scanning angle α; (b) Simulated 2-D beam scanning patterns for different additive phase shift p and scanning angle α in half yz-plane (azimuth plane).

Figures 3(a) and (b) show the simulated 3-D radiation patterns and simulated 2-D beam scanning patterns of the shift p (additive phase difference between adjacent array elements) and scanning angle α [beam-forming scanning for different additive phase angles of the 4-elemet array antenna form +y (0 degree) to –y (180 degrees)] at 28 GHz. For the condition of p = 0o and α = 90o shown in Figure 3(a), it can be seen that an broadside and fan-shape beam toward +z direction with a wide 3 dB beamwidth over 180 degrees can be formed in xz-plane (elevation plane). As shown in Figure 3(b), it can be seen that the 2-D beam scanning coverage can be achieved about 120o with different additive phase shifts p between adjacent array elements, wherein the minimum required 6 dBi realized gain is defined by the maximum realized gain 8 dBi of the beamforming beam with p = 40o and α = 80o minus 2 dBi.

Figure 4. (a) Over-the-air beam scanning performance testing platform for the proposed 4-element array antenna with a software-defined digital beamforming receiver; (b) Measured SNR of the proposed 4-element array antenna with beam scanning angles α from 30 degrees to 150 degrees at 28 GHz. Figure 4(a) shows an over-the-air (OTA) beam-scanning measurement platform for testing beamforming performance of the 4-element array antenna. As shown in Figure 4(a), the

transmitting part mainly consists of a vector RF source, a Tx up-converter and a horn antenna. The receiving part mainly consists of the 4-element array antenna, a 4-path Rx down-converter and a 4-port oscilloscope controlled by a software-defined digital beamforming receiver which is jointly developed by Antennas and Communications Labs. of National Sun Yat-sen University in Taiwan. The testing distance is 50 cm. Figure 4(b) shows the measured scanning SNR (signal-to-noise ratio) results with different phase shifts p at 28 GHz. It can be seen that the maximum SNR is about 28.5 dB at α = 75o. In the desired coverage area form α = 30o to 150o (total 120o), the measured SNR variation is about 2 dB which agrees with the simulation trend shown in Figure 3(b). More results about the OTA beam-scanning measurement platform and measured performance analysis of the proposed 4-element array antenna will be presented and discussed in the presentation.

IV. CONCLUSION

A highly-integrated wideband mmWave slot-type array antenna for 5G mobile phones has been proposed, fabricated, and studied. An array element of parallel-connected dual open-slots is proposed for forming the array antenna. A 4-element wideband array antenna occupying a small layout area of 21.3×1.64 mm2 and having a wide bandwidth of 23.9~29.5 GHz for covering the whole 26 GHz and 28 GHz bands (n258 and n257 bands) to fulfill different 5G NR spectrum allocations of different countries at lower mmWave frequencies has been demonstrated. The measured results show that wide-coverage broadside fan-shape beamforming patterns and wide beam scanning coverage about 120 degrees can be achieved. With the obtained results, the proposed wideband mmWave array antenna would be promising for future 5G mobile phone applications.

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