Compact and Broadband Substrate Integrated Dielectric Resonator Antenna Suitable for 5G Millimeter-Wave Communications

To date, few broadband DRAs can cover n257, n258, n260, and n261 bands with a small physical footprint (e.g., < <inline-formula> <tex-math notation="LaTeX">$0.4\lambda _{0}\times 0.4\lambda _{0}\times 0.15\lambda _{0}$ </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free-space wavelength at 28GHz). This article proposes a compact and broadband substrate-integrated dielectric resonator antenna (SIDRA) suitable for 5G millimeter-wave band applications. Four operating modes from three resonators, including TE111 and TE131 modes from the DR, slot mode from the H-shaped feeding slot, and patch mode from the inserted ring patch, are excited to achieve a bandwidth of 61.9% (24-45.5 GHz) with a compact size of <inline-formula> <tex-math notation="LaTeX">$0.37{\lambda }_{0}\times 0.37{\lambda }_{0}\times 0.125{\lambda }_{0}$ </tex-math></inline-formula>. The proposed DRA can be extended to an array with <inline-formula> <tex-math notation="LaTeX">$\sim 0.5 \lambda _{0}$ </tex-math></inline-formula> element interval to obtain wide-angle beam scanning capability. A <inline-formula> <tex-math notation="LaTeX">$1\times 4$ </tex-math></inline-formula> SIDRA array was simulated, achieving beam-scanning area of ±45° and ±32° at the frequencies of 28/39 GHz. Further, the DRA array was fabricated and tested. It offers a measured 10-dB bandwidth (|S11| ≤ −10 dB) of ~60.4% (23.5-43.7 GHz), in which the gain varies between 10.1 to 12.5 dBi.


I. INTRODUCTION
I N RECENT years, millimeter-wave antennas are widely used to provide high-data-rate communications in fifthgeneration (5G) technologies [1], [2].Generally, millimeterwave antennas should be designed based on the following considerations: 1) Wide bandwidth.Several commercial bands in millimeter-wave region are introduced, such as n257, n258, n260, n261, etc., ranging from 24.25 to 43.5 GHz.The antennas which can cover the whole bandwidth would become very attractive.2) Compact size.To achieve wide angular coverage and avoid grating lobes, the element spacing of beam-scanning arrays are required to be around 0.5 λ 0 (λ 0 is the free-space wavelength at 28GHz) [3], [4].Therefore, the antenna element should be compact enough (<0.4λ 0 ×0.4λ 0 ) to support the array design requirements.3) High efficiency.The adoption of massive multiple-input multiple-output (Massive-MIMO) technology in millimeter-wave bands results in a significant rise in energy consumption.To improve energy efficiency, it is essential for the antenna to possess high antenna efficiency.
It is noted that all the aforementioned antennas are metallic antennas, which suffer from the defect of high ohmic losses in the millimeter-wave band, therefore having relatively low efficiency.To solve this issue, dielectric resonator antenna (DRA), which has higher antenna efficiency, is considered as a promising candidate [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32].In [20], a 2×2 DRA array can achieve a bandwidth of 25-40 GHz by using a novel but bulky feeding structure.In [26], a low profile and decoupled connected DRA array is presented.However, it can only cover a bandwidth of 22.5-30 GHz.To the authors' knowledge, there is currently no reported DRA design that can provide a wide bandwidth spanning from 24 GHz to 43 GHz, while maintaining a compact size suitable for wideangle beam-scanning applications.
To solve this issue, a compact and broadband substrateintegrated DRA (SIDRA) is proposed in this article.Compared to ceramic DRA, SIDRA is more convenient for processing and assembly [33], [34].By properly combining the DR, slot, and ring patch, the proposed antenna can produce a total of four resonant modes.Specifically, the fundamental mode TE 111 and higher order mode TE 131 of the DRA are excited at 32 GHz and 43.5 GHz, respectively.The H-shape slot feeding structure also functions as a resonant radiator at 25 GHz, whereas the ring patch produces an additional mode at around 38 GHz.The antenna can, therefore, cover the whole millimeter-wave band of 24-43 GHz with a small physical footprint of 0.37 λ 0 ×0.37 λ 0 ×0.125 λ 0 .It can also be extended to an array with ∼0.5 λ 0 element interval to obtain wide-angle beam scanning capability.

II. MULTI-MODE HYBRID DRA A. ANTENNA CONFIGURATION
Fig. 1 illustrates the configuration of the proposed SIDRA.It is a PCB-based multilayered structure containing three substrate layers and two prepreg layers.The square SIDRA is directly constructed in Substrate 1, having a side length of a.A square-ring patch is printed on the upper surface of Substrate 2, whose outer length is given by a m and inner length by a n .An H-shaped slot is etched on the upper metallic layer of Substrate 3, while an microstrip line is printed on the bottom surface.They together server as a feeding structure to excite the above SIDRA.

B. OPERATING PRINCIPLE
To investigate the operation of the proposed SIDRA, the evolution of the antenna structure is studied.Fig. 2 shows the configurations of the three antennas, while Fig. 3 depicts  the corresponding simulated reflection coefficients.The fullwave solver ANYSIS HFSS is used for simulation.It is found that antenna A have two resonances at 31 GHz and 44 GHz, which is contributed by the DRA's fundamental TE 111 mode and the high-order TE 131 mode, respectively.To include the feeding slot mode, Antenna B is achieved by adjusting the size of the H-shaped slot, mainly shortening its length.Through the adjustment, the slot mode is shifted up to around 25 GHz, which is close to the TE 111 mode and the bandwidth has potential to be enhanced.However, even though the three resonant modes are excited within the target band, it is difficult to optimize the impedance Here it is important to note that the adopted ring patch has two advantages.First, it has a very small size (much smaller than a square patch), thus having little influence on the other resonators.Second, it does not increase the overall antenna dimension as it is inserted below the DRA.
To further investigate the operation mechanism of the SIDRA, the E-field patterns of the antenna at different resonant frequencies in the xoy and yoz planes are illustrated in Fig. 4. Fig. 4(a) depicts the E-field patterns at 25 GHz, which can be seen that a strong E-field distribution is concentrated around the H-shaped slot, suggesting that the resonance is generated by the slot.In Fig. 4(b) and Fig. 4(d), it can be found that the E-field distribution is concentrated on the upper surface of the DRA at 32 GHz and 43.5 GHz, and the corresponding shapes of the field pattern are consistent with the DRA's fundamental TE 111 mode and higher-order TE 131 mode.Fig. 4(c) shows the E-field distributions at 38GHz.It is quite strong around the ring patch.On this basis, the mode around 38 GHz can be defined as a ring patch mode.

C. PARAMETRIC STUDIES
In order to provide a clear demonstration of how the different structural parameters affect the antenna, parametric studies have been performed.The impact of the DR on the antenna was also investigated.The reflection coefficients against a and h 1 are simulated in Fig. 5(b) and (c).As a and h 1 increase, the resonant frequencies around 32 GHz (TE 111 mode) and 43.5 GHz (TE 131 mode) shift downward apparently as the other two resonances remain unchanged.This is reasonable because both the second and fourth modes are produced by the DRA.It is worth noting that the influence of a on the resonance of TE 131 mode is slightly stronger than that on TE 111 mode, while h 1 has the opposite influence on the two resonances.
A parametric study of the side length of ring patch was carried out in Fig. 5(d).It can be learned that the variation of a m significantly affects the resonant frequency around 38 GHz.This phenomenon confirms that the third resonant mode is mainly generated by the ring patch.

D. DESIGN GUIDELINE
Based on the operating principle mentioned above, a concise design guideline is summarized as follows: 1. Determining the dimensions of the square SIDRA.The initial values of the SIDRA is determined to ensure the resonances of TE 111 and TE 131 modes radiate at target frequencies.2. Inserting the ring patch to extend the bandwidth.The small ring patch has negligible effect on the DR operation.It can be then inserted into the DRA with its size determined to enhance the antenna bandwidth.3. Tuning the length of slot.In addition to tuning the impedance matching, the H-shaped slot is also operating as a resonator.The slot size should be carefully tuned for both the mode resonating and impedance matching.4. Optimizing the antenna structure.The parameters of antenna can be further optimized to achieve a good performance.The radiation patterns at low (26 GHz), middle (32 GHz), and high (38 GHz) frequencies are presented in Fig. 6(b)-(d).The simulated results demonstrate that the antenna element can achieve symmetrical radiation patterns on both the planes across the wide operating band.Within the 3-dB beamwidths, the cx-pol (cross-polarization) levels remain below -32 dB in the E-plane and -41 dB in the H-plane.

III. 1×4 HYBRID ANTENNA ARRAY
The proposed SIDRA is extended to a 1×4 array as shown in Fig. 7 since a 1×4 array is currently the most commonly used millimeter-wave antenna solution for mobile terminals.To achieve a wide-angle coverage, the element spacing is set as 5.5mm (∼0.51λ 0 at 28 GHz).The plated-throughvias surrounding the antenna elements are used to reduce the mutual coupling.Fig. 8 shows the simulated radiation efficiency of the proposed antenna array, which is varying from 83.6% to 92.5%.Fig. 9 shows the simulated active |S 22 |s and mutual couplings between different ports of the antenna array, where we can learn that the mutual coupling is lower than -15 dB within the band.The active parameters are better than -10 dB at the scanning angle of 0 • (@28 GHz) and better than -8.5 dB with the scanning area of ±25 • .When scanning angle increases up to 45 • , the active |S 22 | is better than -6.5 dB.Fig. 10 illustrates the simulated 2D beam-scanning performance of the DRA array by introducing a stepwise phase shift between the elements at 28 / 39 GHz.In H-plane, the observed scan losses are lower than 1 dB with sidelobe levels better than 5 dB within the scanning area of ±45 • and ±32 • at 28 GHz and 39 GHz.
The proposed design concepts were verified by fabricating and measuring the antenna array.Fig. 11 (a) and  The simulated gain varies from 10.8 dBi to 13.5 dBi, and the measured array gain varies between 10.1 dBi and 12.5 dBi.
Fig. 12 illustrates the radiation patterns of the SIDRA array, showing both simulated and measured results.The measured main lobes match well with the simulated ones at all three frequencies.In the H-plane, the measured maximum sidelobe levels are given by -12.5 dB, -12.4 dB, and -16.7 dB.The measured X-pol levels within the 3-dB beamwidths are better than -29.2 dB/-25.2dB/-29.7 dB in the E-plane and -29 dB/-27.4dB/-27 dB in the H-plane at 26/32/38 GHz, respectively.Suffer from the fabrication tolerances as well as the measurement error, there are some slight discrepancies between the simulated and measured results.
Table 1 presents a comparative analysis between the proposed SIDRA and previously reported millimeter-wave DRAs, highlighting the contributions of this paper.It can be found that majority of the DRA designs have limited bandwidths and cannot achieve the full band coverage of 24.25-43.5GHz.In [20], although the DRA obtains a wide bandwidth of 42% (covering 25-40 GHz), it has Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.a complicated feed structure which makes it hard to be extended to an array design.The proposed SIDRA element has comparable compact size and radiation efficiency with the competitors, but demonstrating a much wider bandwidth.Furthermore, the proposed antenna can be extended to an array to support wide-angle beam scanning.Moreover, it can be fabricated by using standard PCB process, which can mitigate the positioning error and improve the integration level, enabling the applications in commercialized batch fabrication at millimeter-wave band.

IV. CONCLUSION
In this article, a compact and broadband SIDRA and its array design have been constructed and investigated for 5G millimeter-wave applications.A wide bandwidth, which can cover the whole n257, n258, n260 and n261 VOLUME 4, 2023 987 Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
(24.25-43.5 GHz) bands, has been realized by exciting four different operating modes.Within the bandwidth, the SIDRA can also achieve stable radiation patterns and gain.The proposed SIDRA element has been further extended to a 1×4 antenna array.The simulated results reveal that the array can achieve wide beam-scanning angles at target frequencies.
The measured |S 11 |s, gains and radiation patterns show tolerable differences compared to the simulated results.It can be concluded that the proposed design is valuable for millimeter-wave applications.

FIGURE 2 .
FIGURE 2. Structure evolution of the proposed SIDRAs.

FIGURE 3 .
FIGURE 3. Simulated |S11|s for the antennas with different structures.
Fig. 5(a) shows the simulated |S 11 | corresponding to different lengths (L s1 ) of the H-shaped slot.With reference to the figure, the first resonant mode decreases from 26 GHz to 23.5 GHz as L s1 increases from 1.5 to 1.9 mm while the other resonances remain stable.This phenomenon also indicates that the resonance around 25 GHz is closely related to the H-shaped slot.

Fig. 6
Fig. 6 depicts the simulated performance of the DRA element.The reflection coefficient and gain are shown in Fig. 6(a).With reference to the figure, the impedance bandwidth (|S 11 | < -10 dB) is given by 61.9% (24-45.5 GHz), and the gain is stable between 5.3 dBi and 8.1 dBi within the impedance bandwidth.The radiation patterns at low (26 GHz), middle (32 GHz), and high (38 GHz) frequencies are presented in Fig.6(b)-(d).The simulated results demonstrate that the antenna element can achieve symmetrical radiation patterns on both the planes across the wide operating band.Within the 3-dB beamwidths, the cx-pol (cross-polarization) levels remain below -32 dB in the E-plane and -41 dB in the H-plane.

FIGURE 7 .
FIGURE 7. Structures of the proposed SIDRA array.(a) Top view.(b) Side view.

FIGURE 8 .
FIGURE 8. Simulated radiation efficiency of the proposed antenna array.

FIGURE 9 .
FIGURE 9. Simulated active S-parameters and mutual couplings between the ports of the antenna array.

FIGURE 11 .
FIGURE 11.The prototype of the 1×4 DRA array with its measured and simulated results.(a) Top view of the array prototype.(b) Bottom view of the array prototype.(c) Measured and simulated gains and |S11|s of the array prototype.