Dielectric Resonator Antenna Arrays for 5G Wireless Communications
Global deployments of 5G networks are on a rapid pace and are expected to deliver a 100x increase in network capacity compared to 4G. To expand network capacity, the 5G NR air interface enables diverse spectrum in both the sub-6 GHz and mmWave frequency bands. The additional spectrum enabled by 5G brings new challenges for product design and global deployment, particularly in the mmWave frequency range. Customer premise equipment (CPE) solutions that operate in the mmWave frequency band are required to bring 5G fixed wireless access (FWA) to urban and suburban landscapes. To achieve the high data rates and low latencies promised by 5G, the antenna system becomes a crucial component in the overall system design. The Antenna Company is developing mmWave active antenna arrays for CPE applications that utilize novel materials and advanced antenna design strategies to address the performance limitations of conventional printed circuit board (PCB) solutions. This article discusses the performance benefits of utilizing a 64-element dielectric resonator antenna (DRA) array.
DRA BENEFITS FOR 5G
The new generation of 5G wireless communications is designed to provide mobile users with fiber-like data speeds, low latency and high signal ﬁdelity. Industry-wide development efforts to enable these performance breakthroughs include advanced channel coding techniques, massive MIMO and mobile mmWave. Breakthroughs in antenna system design are crucial for the global use of mmWave spectrum. In addition to providing radio coverage, antenna systems must be able to reconﬁgure the radiation pattern characteristics to meet the dynamics in 5G wireless communications.1
Advances in material science and manufacturing make DRA technology a valid solution for the development of commercial array antennas. DRAs share several features with their counterpart, patch antennas; they are compact and are easy to integrate with active electronics. However, DRAs are more efﬁcient than patch antennas at mmWave frequencies due to the high conduction loss. In addition, DRAs can provide more design freedom, compared to patch antennas, in relation to the geometry and materials forming the basic dielectric resonating structure. Different feeding techniques, design procedures and DRA structures can be found in review papers.2-3 Long et al. systematically studied the radiation characteristics of different DRAs and their potential for mmWave applications.4 It is shown that DRAs are compact, lightweight, cost-effective and feature broader bandwidth (BW) characteristics and large scanning angles up to ±60 degrees and beyond, as compared to patch antennas.5-6
The BW of phased array antennas is mostly determined by the active reﬂection coefﬁcient during beam steering that affects the scan-loss characteristics of the antenna array. In addition, the shape of the embedded element pattern plays an important role. The mutual coupling can degrade the pattern by modifying the embedded pattern shape and the realized excitation coefﬁcients.
An 8x8 array of dielectric resonator antennas fed by slot antennas is proposed for mmWave 5G wireless communications. The total radiation efﬁciency is modeled as a function of frequency and beam steering. It is shown that the total radiation efﬁciency is higher than 80 percent over the frequency band from 26 to 34 GHz and for scan angles up to 60 degrees. The DRA can perform beam steering over wide BW with a scan-loss comparable to the array factor of an ideal source. In addition, the DRA array is 20 percent smaller in size compared to conventional half-wavelength spaced antenna arrays.
In this article, attention is given to the optimal design of hybrid DRA/slot antennas. It will be shown that the antenna impedance BW can be easily tuned in such a way as to synthesize a wide- or multi-band frequency response.
ANTENNA ARRAY DESIGN
Individual Antenna Element Design
A single array antenna element consists of a slot antenna fed by proximity coupling via a microstrip transmission line. The dielectric material for the transmission line is RO4308 with relative permittivity (εr) of 3.6. The slot then feeds a square dielectric rod, as illustrated in Figure 1. In this design a dielectric material with εr = 11.3 is used in the analysis. With a proper selection of dimensions similar to Keyrouz and Caratelli,5 the slot antenna radiates properly and excites the proper modes in the dielectric rod (Transverse Magnetic modes). A detailed mode analysis can be found in Long et al.4
By using a fork-like microstrip line to feed the aperture slot (see Figure 2), the impedance can be well matched to 50 Ω with a broadband reﬂection coefﬁcient better than −10 dB over the frequency range from 26 to 30 GHz (see Figure 1). In particular, the antenna element is fitted with a suitable mini-SMP connector, as shown in Figure 3. Notice that the considered DRA element displays a radiation pattern that is mostly conﬁned in the half-sphere above the relevant ground plane.
Antenna Array Scan Properties
The aperture-fed DRA detailed in the previous section is placed in a uniform 64-element array consisting of 8x8 elements, as shown in Figure 4. The inter-element spacing is about half-wavelength at 28 GHz. The objective is to examine the array performance while beam steering over the operational range of frequencies.
For a two-dimensional planar array, the beam steering angle (θ, φ) can be controlled by setting certain phase values at the (m, n)th port according to Equation 1, in which m and n stand for row and column, dx and dy stand for the spacing in x- and y- direction, respectively.9
As shown in Figure 5, the phased DRA array features wideband characteristics. The total radiation efﬁciency is better than 80 percent over the frequency band between 26 and 34 GHz along the H-plane. Also, the DRA array displays nearly flat efﬁciency characteristics over a wide scan range. Such high performance is related to the fact the DRA radiating elements are wideband and the electromagnetic ﬁeld is conﬁned within the relevant dielectric rod. As a result, the inter-element coupling is much smaller as compared to patch array antennas. A comparable patch array antenna with equivalent aperture and inter-element spacing displays a much narrower band performance with a poor efﬁciency away from the central resonant frequency, as it appears from Figure 6. The patch array elements are fed via slots. The dielectric laminate on which the patch antenna elements are printed is RO4308 with relative permittivity of εr = 3.6.
The active reflection coefficient of the array is significantly more complicated to evaluate as it requires the evaluation of the mutual coupling data between the activated array element and the adjacent elements. Based on the derivation in Pozar,7 for a two-dimensional planar array, the active reflection coefficient Γ can be calculated by Equation 2, where Sm,n stands for the mutual coupling coefficient between the m-th and n-th ports. As shown in Figure 7, the broadband characteristics of the proposed DRA array, as compared to the equivalent structure based on patch antenna elements, are also validated in active mode, that is while beam scanning. The considered array structures feature equivalent inter-element spacing and aperture.
In Figure 8, the scan-loss characteristics along both the H- and E-plane are evaluated. From the obtained results, it can be inferred that the scan loss along the H-plane follows the array factor of an ideal cosine source. The performance is just slightly degraded along the E-plane. This difference is mainly due to the asymmetry of the average embedded pattern of the array. On the other hand, a signiﬁcant advantage is related to the fact that the array performance is stable over a very large frequency band.
As it appears in Figure 9, the proposed DRA array displays robust performance also against manufacturing tolerances related to the misalignment or misplacement of the radiating structure on the circuit board containing the relevant feeding network. This is important to ensure reduced sensitivity of the array to non-idealities typically encountered in mass-production assembly lines.
The antenna impedance bandwidth can be easily tuned in such a way to synthesize a wide- or multi-band frequency response by the combination of DRAs and radiating slots. The proposed DRA array achieves a total radiation efﬁciency better than 80 percent in the band from 26 to 34 GHz and large scanning angles compared to a conventional half-wavelength spaced patch antenna array. With the advances in material science and manufacturing techniques, DRA technology can become a cost-effective, high performance solution for 5G wireless communications.
- IEEE 5G, “IEEE 5G and Beyond Technology Roadmap White Paper,” 2017, https://futurenetworks.ieee.org/images/files/pdf/ieee-5g-roadmap-white-paper.pdf.
- K. W. Leung, E. H. Lim and X. S. Fang, “Dielectric Resonator Antennas: From the Basic to the Aesthetic,” Proceedings of the IEEE, Vol. 100, No. 7, 2012, pp. 2181–2193.
- A. Petosa and A. Ittipiboon, “Dielectric Resonator Antennas: A Historical Review and the Current State of the Art,” IEEE Antennas and Propagation Magazine, Vol. 52, No. 5, October 2010, pp. 91–116, http://ieeexplore.ieee.org/document/5687510/.
- S. A. Long, M. W. McAllister and L. C. Shen, “The Resonant Cylindrical Dielectric Cavity Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 31, No. 3, 1983, pp. 406–412.
- S. Keyrouz and D. Caratelli, “Dielectric Resonator Antennas: Basic Concepts, Design Guidelines, and Recent Developments at Millimeter-Wave Frequencies,” International Journal of Antennas and Propagation, Vol. 2016, pp. 1–20, www.hindawi.com/journals/ijap/2016/6075680/.
- M. Simeoni, R. Cicchetti, A. Yarovoy and D. Caratelli, “Plastic-Based Supershaped Dielectric Resonator Antennas for Wide-Band Applications,” IEEE Transactions on Antennas and Propagation, Vol. 59, No. 12, December 2011, pp. 4820–4825.
- D. M. Pozar, “The Active Element Pattern,” IEEE Transactions on Antennas and Propagation, Vol. 42, No. 8, August 1994, pp. 1176–1178.