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


Abstract and Figures

This paper proposes a bandwidth-improved cavity-backed unit cell slot subarray antenna fed by a gap ridge waveguide for Line-of-Sight (LoS) 5G mmWave applications. The proposed subarray is comprised of three layers. The top layer is the antenna layer in a 2 × 2 slots configuration, the cavity layer is in the middle and the ridge gap waveguide feeding is in the bottom layer. We propose to enhance the bandwidth of the unit cell subarray antenna by modifying the coupling slot in the cavity layer. The main advantage of the proposed method is that the bandwidth enhancement can be achieved without adding an extra tuning pin in the cavity layer, which eases mass production. The proposed structure is simulated with the CST software in an infinite array environment. The simulation results show that the S 11 is below -10 dB over 24 − 31.5 GHz frequency band covering a 27% bandwidth and the directivity is approximately 33.8 dBi when evaluated as a 16 × 16 element slot array. It is envisaged that the method can be advantageous for bandwidth enhancement of a practical cavity-backed slot array antenna.
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This article have been accepted for presentation in ISAP 2019, XiAn, China.When citing this work, cite the
original published paper -
Wideband Cavity-Backed Slot Subarray Fed by Gap Ridge
Waveguide for 5G mmWave Base Station
Wai Yan Yong, Abolfazl Haddadi, Alireza Bagheri, Thomas Emanuelsson, Andr´es Alay´on
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 unprecedented
unlicensed bandwidth has gained the support from governments, the industry and academia. High-gain beam
antennas 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 the
waveguide-fed slot arrays [3, 4] have been presented. However, in the case of SIW, the proposed antennas
suffer from high losses due to the presence of a dielectric substrate. Moreover, the radiation efficiency can be
deteriorated and leakage can occur since transmission line design at this frequencies is more sensitive to the
proper design of via holes that are prone to providing deficient shielding [5]. While waveguide-fed slot arrays
manage to resolve the problem with the SIW technology, the key challenge of these waveguide-fed antennas is
that they require a good electrical contact between the feeding and the radiating layers [6]. Hence, a much more
complex manufacturing process is required to ensure good electrical contact among the layers incurring in high
fabrication costs.
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Figure 1: Exploded view of the proposed 2×2subarray antenna (a) front, and (b) back views
A solution overcoming the problems of unsatisfactory electrical contacts as well as the problems due to
the mechanical assembly of a waveguide-fed slot array can be found in the recently introduced gap waveguide
technology [7]. Gap waveguide technology is designed using the periodic artificial magnetic conductor (AMC) to
control the propagation of waves in the desired directions between two PEC plates, i.e., propagation is allowed
in 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 parallel
plate structure.[8]. Recently, several gap waveguide-fed array antennas have been developed at the mmWave
band. However, these antennas usually support a bandwidth of around 15 20% [9, 10, 11]. One of the
recently proposed techniques to enhance the bandwidth performance of the gap waveguide-fed array antenna
is to increase the number of tuning pins in the cavity layer to provide more capacitive tuning on the cavity
layer leading to a wider impedance bandwidth matching [12]. This technique has resulted in a bandwidth of
around 30%. However, the main drawback of the proposed solution is that the dimensions and the positions
of the additional tuning pins are fully determined through numerical simulations [12], which requires heavy
computational burden. Furthermore, the additional pins also result in a higher fabrication costs which is
unfavorable for the industrial fabrication of the antennas. In this paper, we propose an alternative solution for
the bandwidth enhancement of the cavity backed slot array antenna fed by gap waveguide without the need of
adding 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 comprises
3 layers. The top, radiating layer is designed as a 2 ×2 configuration of radiating slots. These radiating slots
are backed by an air-filled cavity in the middle layer. The bottom layer is the feeding layer, which design is
based 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 distance
between 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 conventional
cavity-backed slot array antenna, a rectangular slot is used. The main function of this rectangular coupling
slot is to maximize the energy coupled from the feed gap waveguide into the cavity layer [6]. Therefore, in this
paper, we propose to modify the rectangular slots (RSL) into a ’bow-tie’ shaped slot (BTSL) for bandwidth
enhancement. By replacing the RSL with the BTSL, the coupling slot behaves like a double-ridge slot. This
feature allows the cut-off frequency in the dominant modes to shift into a higher frequency and the resonant
frequency of the next higher order modes is altered to a lower frequency. In addition, the T-shaped cavity
tuning pin is used for impedance matching and suppressing higher order modes. Hence, by proper modification
of the coupling slot and the cavity tuning pin, the bandwidth performance of the subarray antenna has been
Figure 2: Comparison of the simulated reflection coefficient S11 of the conventional rectangular cavity slot (RSL) and modified
bow-tie cavity slot (BTSL). fis 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 various
frequencies f, where G0is the antenna gain in dBi, θis the polar angle in degrees.
3. Simulation Results
The simulation and optimization of the proposed unit cell subarray has been performed using the Computer
Simulation Technology (CST) software. The simulations have been performed assuming the infinite array model
with periodic boundary conditions. The 2 ×2 slot subarray is excited by a waveguide port at the ridge gap
waveguide in the bottom feeding layer. Fig. 2 illustrates the comparison of the simulated reflection coefficient
of the proposed unit cell subarray with conventional RSL and the modified BTSL in the cavity layer. As can
be seen, the conventional RSL has a 10 dB relative bandwidth of 20.6% (24 29.5 GHz), which has been
considerably improved to the relative bandwidth of 27% (24 31.5 GHz) by using the modified BTSL in the
cavity layer.
Fig. 3 illustrates the normalized far-field gain of the proposed unit cell slot array simulated over the operating
bandwidth at 2431.5 GHz for the E-plane and the H-plane in subplots (a) and (b),respectively. The simulated
gain of the proposed unit cell slot array is around 15.6 dBi at the center frequency of 27 GHz. The radiation
patterns 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 antenna
over 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 over
24 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 shaped
coupling 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. The
radiation 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 array
antenna is less than 15 dBi over the operating bandwidth. The proposed unit cell is a promising subarray
element for a fixed beam array antenna. In future work, we will further investigate the bandwidth improvement
by combining the addition of more tuning pins and our proposed modifying the cavity coupling slot solution.
This project has received funding from the European Unions Horizon 2020 research and innovation programm
under the Marie Sklodowska-Curie grant agreement No. 766231 WAVECOMBE H2020-MSCA-ITN-2017.
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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 various
frequencies. D0/Dmax denotes the normalized antenna gain in dBi and θis the polar angle in degrees.
Figure 5: Computed directivity D0of the designed 16 ×16 slot array antenna as a function of frequency f.
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