This article have been accepted for presentation in ISAP 2019, XiAn, China.When citing this work, cite theoriginal published paper - https://ieeexplore.ieee.org/abstract/document/8963135

Wideband Cavity-Backed Slot Subarray Fed by Gap RidgeWaveguide for 5G mmWave Base Station

Wai Yan Yong, Abolfazl Haddadi, Alireza Bagheri, Thomas Emanuelsson, Andres AlayonGlazunov

aUniversity of Twente, Department of Electrical Engineering, Enschede, Netherlands.bGAPWAVES AB, Gothenburg, Sweden.

cChalmers University of Technology, Department of Electrical Engineering, Gothenburg, Sweden.

Index Terms— Gap Waveguide, Slot Array, 5G, Millimeter Wave

1. Introduction

5G communication systems are envisioned to provide Gbps peak data rates to multiple users simultaneously.To realize this arduous vision, moving toward the millimeter-wave (mmWave) spectrum offering unprecedentedunlicensed bandwidth has gained the support from governments, the industry and academia. High-gain beamantennas are required to compensate for the high propagation path loss in the mmWave bands. To this end,numerous antenna designs employing substrate integrated waveguide (SIW) technologies [1, 2] as well as thewaveguide-fed slot arrays [3, 4] have been presented. However, in the case of SIW, the proposed antennassuffer from high losses due to the presence of a dielectric substrate. Moreover, the radiation efficiency can bedeteriorated and leakage can occur since transmission line design at this frequencies is more sensitive to theproper design of via holes that are prone to providing deficient shielding [5]. While waveguide-fed slot arraysmanage to resolve the problem with the SIW technology, the key challenge of these waveguide-fed antennas isthat they require a good electrical contact between the feeding and the radiating layers [6]. Hence, a much morecomplex manufacturing process is required to ensure good electrical contact among the layers incurring in highfabrication costs.

[] 3 []Figure 1: Exploded view of the proposed 2 × 2 subarray antenna (a) front, and (b) back views

A solution overcoming the problems of unsatisfactory electrical contacts as well as the problems due tothe mechanical assembly of a waveguide-fed slot array can be found in the recently introduced gap waveguidetechnology [7]. Gap waveguide technology is designed using the periodic artificial magnetic conductor (AMC) tocontrol the propagation of waves in the desired directions between two PEC plates, i.e., propagation is allowedin certain directions while it is suppressed in others [8, 9]. Indeed, by introducing an air gap with a size of

smaller than λ/4 between the PEC-PMC plates, no waves can propagate in between the PEC-PMC parallelplate structure.[8]. Recently, several gap waveguide-fed array antennas have been developed at the mmWaveband. However, these antennas usually support a bandwidth of around 15 − 20% [9, 10, 11]. One of therecently proposed techniques to enhance the bandwidth performance of the gap waveguide-fed array antennais to increase the number of tuning pins in the cavity layer to provide more capacitive tuning on the cavitylayer leading to a wider impedance bandwidth matching [12]. This technique has resulted in a bandwidth ofaround 30%. However, the main drawback of the proposed solution is that the dimensions and the positionsof the additional tuning pins are fully determined through numerical simulations [12], which requires heavycomputational burden. Furthermore, the additional pins also result in a higher fabrication costs which isunfavorable for the industrial fabrication of the antennas. In this paper, we propose an alternative solution forthe bandwidth enhancement of the cavity backed slot array antenna fed by gap waveguide without the need ofadding any additional tuning pins in the cavity layer.

2. Antenna Design

Fig. 1 illustrates a view of the proposed slot array unit cell in perspective. The proposed structure comprises3 layers. The top, radiating layer is designed as a 2 × 2 configuration of radiating slots. These radiating slotsare backed by an air-filled cavity in the middle layer. The bottom layer is the feeding layer, which design isbased on the gap waveguide technology. The unit cell dimension of the proposed subarray is 18.3 × 19 mm2

in the E-plane and the H-plane, respectively. A single slot has the dimensions of 3.6 × 6.4 mm2. The distancebetween every two slots in x- and y-directions are 9.15 mm and 9.5 mm, respectively. Both distances in x-and y-directions are less than one wavelength of the 31.5 GHz to avoid high grading lobes. In the conventionalcavity-backed slot array antenna, a rectangular slot is used. The main function of this rectangular couplingslot is to maximize the energy coupled from the feed gap waveguide into the cavity layer [6]. Therefore, in thispaper, we propose to modify the rectangular slots (RSL) into a ’bow-tie’ shaped slot (BTSL) for bandwidthenhancement. By replacing the RSL with the BTSL, the coupling slot behaves like a double-ridge slot. Thisfeature allows the cut-off frequency in the dominant modes to shift into a higher frequency and the resonantfrequency of the next higher order modes is altered to a lower frequency. In addition, the T-shaped cavitytuning pin is used for impedance matching and suppressing higher order modes. Hence, by proper modificationof the coupling slot and the cavity tuning pin, the bandwidth performance of the subarray antenna has beenimproved.

Figure 2: Comparison of the simulated reflection coefficient S11 of the conventional rectangular cavity slot (RSL) and modifiedbow-tie cavity slot (BTSL). f is the frequency.

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Figure 3: Simulated radiation patterns of the proposed unit cell slot array antenna at (a) E-plane, and (b) H-plane at variousfrequencies f , where G0 is the antenna gain in dBi, θ is the polar angle in degrees.

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3. Simulation Results

The simulation and optimization of the proposed unit cell subarray has been performed using the ComputerSimulation Technology (CST) software. The simulations have been performed assuming the infinite array modelwith periodic boundary conditions. The 2 × 2 slot subarray is excited by a waveguide port at the ridge gapwaveguide in the bottom feeding layer. Fig. 2 illustrates the comparison of the simulated reflection coefficientof the proposed unit cell subarray with conventional RSL and the modified BTSL in the cavity layer. As canbe seen, the conventional RSL has a −10 dB relative bandwidth of 20.6% (24 − 29.5 GHz), which has beenconsiderably improved to the relative bandwidth of 27% (24 − 31.5 GHz) by using the modified BTSL in thecavity layer.

Fig. 3 illustrates the normalized far-field gain of the proposed unit cell slot array simulated over the operatingbandwidth at 24−31.5 GHz for the E-plane and the H-plane in subplots (a) and (b),respectively. The simulatedgain of the proposed unit cell slot array is around 15.6 dBi at the center frequency of 27 GHz. The radiationpatterns at the E- and H-plane of a 16× 16 comprising 4 2× 2 slot subarrays are computed and shown in Fig 4.As can be sene, the side-lobe-level of the 16× 16 array is low which is below −15 dBi for both E- and H-planes.Fig. 5 shown the computed directivity of the proposed slot array antenna with 16 × 16 elements array antennaover 24 − 31.5 GHz. At center frequency of 27.5 GHz, the directivity is around 33.8 dBi

4. Conclusion

A numerical design of a 2×2 unit cell slot subarray based on the ridge gap waveguide feeding operating over24 − 31.5 GHz is presented. The design covers the entire proposed mmWave spectrum for 5G communications.We proposed a modification to the conventional rectangular slot in the cavity layer using a bow-tie shapedcoupling slot for the purpose of bandwidth enhancement. By modifying the coupling slot in the cavity layer,the −10 dB relative bandwidth of the proposed slot array antenna has been increased from 20.5% to 27%.The proposed unit cell slot array shows a high directivity of 15 dBi at the centre frequency of 27.5 GHz. Theradiation pattern of the designed 16 × 16 slot array comprising eight 2 × 2 slot subarrays is also computed.The directivity at 27.5 GHz is approximately 33.8 dBi. Moreover, the side-lobe-level of the 16 × 16 slot arrayantenna is less than −15 dBi over the operating bandwidth. The proposed unit cell is a promising subarrayelement for a fixed beam array antenna. In future work, we will further investigate the bandwidth improvementby combining the addition of more tuning pins and our proposed modifying the cavity coupling slot solution.

Acknowledgment

This project has received funding from the European Unions Horizon 2020 research and innovation programmunder the Marie Sklodowska-Curie grant agreement No. 766231 WAVECOMBE H2020-MSCA-ITN-2017.

References

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Figure 4: Computed radiation patterns of the proposed 16 × 16 slot array antenna for (a) E-plane, and (b) H-plane at variousfrequencies. D0/Dmax denotes the normalized antenna gain in dBi and θ is the polar angle in degrees.

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Figure 5: Computed directivity D0 of the designed 16 × 16 slot array antenna as a function of frequency f .

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