Mechanically Reconfigurable Slotted-Waveguide Antenna Array for 5G Networks

Transcription

1 Mechanically Reconfigurable Slotted-Waveguide Antenna Array for 5G Networks H.R.D. Filgueiras, I. F. da Costa, Arismar Cerqueira S. Jr. Laboratory WOCA, National Institute of Telecommunications (Inatel), 510 João de Camargo Av., Santa Rita do Sapucaí- MG, Brazil, R. A. Santos Federal University of Itajubá (UNIFEI), B P S Avenue, 1303, Itajubá-MG, Brazil, James R. Kelly ICS/5GIC, The University of Surrey, Stag Hill Campus, Guildford, GU2 7XH, UK. j.r.kelly@surrey.ac.uk Abstract This paper proposes a high-gain mechanically reconfigurable antenna array based on a ring-shaped slottedwaveguide antenna for mm-wave applications. A full scanning range in the azimuthal plane is ensured by proper mechanically rotating a metallic jacket, which partially covers the array radiating structure. The technique provides a beamwidth of 37º in the azimuth plane and gain of dbi at the operating frequency of 27.3 GHz. Significantly, unlike a conventional phased array, this approach does not suffer from: scan loss, beam broadening or SLL degradation. Experimental results of the array element and numerical results of the antenna array demonstrate its applicability in 5G cellular networks. Keywords 5G networks, antenna array, beam steering, mmwaves and reconfigurable antennas. I. INTRODUCTION In the last years, the number of cellular users has exponentially increased mainly due to new applications such as high definition video streaming. These new technologies demand higher data throughput and hence bandwidth, which the current mobiles network are unable to support. The fifthgeneration of cellular systems (5G) will aim to satisfy this large demand. Millimeter-wave (mm-wave) technology is one of a number of competing solutions to this problem. The mm-wave bands offer large and mostly un-exploited bandwidth. However, new challenges arise when higher frequencies are used. For instance, the free-space path loss increases, as well as the complexity of systems components, including antennas, amplifiers and filters to name a few. One of the reasons for this additional complexity is that the components size get smaller as the wavelength decreases, demanding high accuracy fabrication processes [1]. Reconfigurable antennas with manageable radiation pattern shapes are of great interest for 5G communication systems, since the capability of maintaining user connectivity, in a highly changeable environment, is highly desirable [2]. According to C. G. Christodoulou et al [3], reconfigurable antennas can be classified into two types: switch-based and non-switch-based reconfigurable antennas. The first type can be based on optical, electrical or thermal switching components. Reconfiguration is achieved by integrating of one of these switches into the antennas radiating element(s) or feeding network [3]. For example, our group recently proposed the first opticallycontrolled reconfigurable antenna for mm-wave in the literature [4]. The second type of reconfigurable antenna identified by Christodoulou et al relies on alternative techniques to achieve reconfiguration of any electromagnetic property. This can be achieved mechanically by applying actuators or motors rather than using switches. This class of antenna is also known as a mechanically reconfigurable antenna, in which such components are used to make the antenna structure by moving part of it in order to achieve the desired beam steering [3]. Alternatively, reconfiguration can be achieved through the use of through the use of tunable electronic materials. Fig. 1. Wireless coverage provided by the proposed reconfigurable SWAA. In this paper, we propose a novel mechanically reconfigurable antenna array based on a ring-shaped slotted /17/$ IEEE

2 waveguide antenna array (SWAA), previously developed by our group [5], and a metallic jacket. It provides a reconfigurable radiation pattern in the 28 GHz band. The antenna could be employed in radio access applications. A full 360 scanning range in the azimuth plane is ensured by proper mechanically rotating a metallic jacket, which partially covers the array radiating structure, as a function of the mobile environment. For instance, the array main lobe can be steered in the direction of the mobile users, as illustrated in Fig. 1. The manuscript is structured in three sections. Section 2 presents the design of the ring-shaped SWAA, as well as its simulated and experimental results. The proposed mechanically reconfigurable antennas array is described in Section 3. Finally, conclusions are drawn in Section 4. diameter and operational wavelength (λ), accordingly to the equation (1) [12]: II. OMNIDIRECTIONAL RING-SHAPED SLOTTED-WAVEGUIDE ANTENNA ARRAY DESIGN Slotted-waveguide antenna arrays (SWAAs) consist of a waveguide having a rectangular or circular cross section and featuring slots milled into their conducting walls. The slots interrupt the current flow. The current must flow around the slot edges, causing them to behave as dipole antennas and, as a consequence, the entire waveguide structure acts as a dipole antenna array. Their main advantages are low loss, simple design and fabrication, linear polarization, low crosspolarization, high power handling and high gain. SWAAs are widely used in radar and airplane applications due to the possibility of fabricating a conformal antenna array. In this work, we propose to use SWAAs for 5G networks. Most SWAAs published in literature are based on rectangular waveguides [6-9]. As an exception, our group has recently reported a high-gain omnidirectional ring-shaped SWAA based on a circular waveguide [5], as shown in Fig. 2. The waveguide is filled by Teflon - a low-loss polytetrafluoroethylene (PTFE) dielectric material - with loss tangent tan δ = 5x10-4 and electrical permittivity ε r=2.1. Its radiation mechanism is similar to that of the omnidirectional leaky-wave antennas based on metallic grating loaded circular dielectric rod structure [10, 11]. For our SWAA, the waveguide feeder was milled. The circular slots are equally spaced in order to guarantee structural symmetry. A metallic reflector fixed by a nylon screw was used to close-off the array radiating structure on the top of the antenna. The periodically rings of dielectric within create a disturbance in the guided electromagnetic waves, thus the energy is gradually radiated from the slots to the air, as the wave propagates along the length of the antenna. The base of the antenna is formed from a conical horn fed by a longitudinal K- connector. This sets up a TM 01 guided mode, which has a rotationally-symmetrical field distribution, with the aiming of ensuring an omnidirectional radiation pattern from the antenna. The main dimensions of the horn are its aperture diameter (d horn) and its axial length (L horn). Additional to the excitation scheme, the separation between the dielectric rings (S Ring) should be adjusted to optimize the omnidirectional pattern in the xy-plane. The antenna directivity depends on the aperture Fig. 2. Omnidirectional ring-shaped SWAA prototype. 2 d horn π D = ε ap (1) λ where ε ap is aperture efficiency. The angle (θ) on the xz-plane of the main beam associated with the ring-shaped SWAA depends on the ring length (L ring) and spacing (S ring). It can be calculated using eqn. (2) [13]: cosθ = β 2π k 0 k 0 s (2) Ring LRing S Ring (3) 0.6 where k 0 is the free space wavenumber and β is the propagation constant of the TM 01 mode propagating along the ring-shaped SWAA. The conical horn antenna was designed to exhibit a realized gain of 10 dbi at 28 GHz, with beamwidth of approximately 33 %. In order to achieve these objectives, d horn = 15 mm and L horn = 8 mm. The designed has been numerically validated by using ANSYS HFSS. The radiation pattern of the ring-shaped SWAA depends on the ring dimensions, which can be calculated according to (2) and (3). In order to ensure that the main lobe of the antenna is located normal to the axis of the antenna (θ = 90º), S Ring should be equal to mm and L ring equal to 6.43 mm. The metallic reflector width was set to 1.8 mm, which corresponds to one-quarter of the guided wavelength at 28GHz,

3 in order to minimize the side lobe level (SLL). Its diameter has been numerically evaluated in order to determine the optimum trade-off between side lobe level (SLL) and main beam gain (G). The best obtained value was d Reflector = 23 mm, which implies SLL=12.60 db and G=12.24 dbi. Fig. 3 presents the radiation patterns for the ring-shaped SWAA, at 28 GHz, in the azimuth (xy) and elevation (xz) planes. The gain variation in xy-plane is only 0.88 db over the full 360. Thus we can conclude that the pattern is omnidirectional pattern with 12.2 dbi of gain, which is ten times higher than a conventional half-wave dipole antenna. This gain enhancement is very important to compensate for the high path losses in the mm-waves range, as mentioned earlier. The antenna pattern is very directive in the xz-plane, having a beamwidth of 6º. III. MECHANICALLY RECONFIGURABLE SWAA This section of the paper presents a mechanical technique for reconfiguring the main lobe direction. This is achieved with the addition of a metallic jacket to the ring-shaped SWAA, presented in the previous section. The main purpose of the jacket is to block part of the radiating area of the antenna and thus making possible to guide the beam in strategic direction. By rotating the jacket around the z-axis it is possible to dynamically control the main beam direction. In this way, we can convert the original ring-shaped SWAA with fixed radiation pattern into an antenna having a mechanically reconfigurable main beam direction and an even higher gain. The major advantage of this technique is the possibility of providing beam steering, over the entire azimuthal plane (360º) without losses or significant changes in the radiation pattern (e.g. scan loss, beam broadening, or SLL degradation), as the beam is steered. Furthermore, the beam steering direction can be very accurately controlled with a high resolution, determined by the accuracy of the steeper motor used to rotate the metallic jacket and, consequently, configure the array radiation pattern. Fig. 5 illustrate the mechanically reconfigurable technique and the proposed SWAA numerical model. Fig. 3. Radiation pattern of the omnidirectional ring-shaped SWAA at 28 GHz in azimuth (xy) plane and elevation (xz) plane. The characterization of the ring-shaped SWAA was made using a Keysight FieldFox Microwave Analyzer N9952A. A comparison between the S 11 experimental and numerical results is presented in Fig. 4, in which it is observed an acceptable level of agreement between the curves. A maximum level of -10 db had been assumed to define the antenna bandwidth, giving rise to a wider measured bandwidth, when compared to that obtained in HFSS. The proposed ring-shaped SWAA prototype provides a bandwidth of approximately 7.7 % from to 29 GHz. (a) Novel design Fig. 5. Mechanically reconfigurable SWAA. (b) Mechanical reconfiguration principle The metallic jacket dimensions needed to be numerically optimized in order to obtain acceptable impedance matching and enable the narrow beam to be proper steered. Firstly, a study was made of the jacket opening size, as a function of number of wavelengths (n) and the jacket thickness (t). Fig. 6 presents the reflection coefficient pertaining to a selection of the most important results from this joint parametric analysis of the parameters n and t. The best results based on minimum value of S 11 are summarized in Table I. Fig. 4. Reflection coefficient of the omnidirectional ring-shaped SWAA.

4 conducted based on comparisons between its results with those of the original SWAA (without the metallic jacket), as reported in Fig. 8. The use of the metallic jacket leads to a reduction in the 10dB return loss bandwidth of the antenna. It also shifts the resonance to a lower frequency. This occurs because the jacket acts like a cavity causing the excitation of additional modes. The fractional 10dB return loss bandwidth of the antenna, incorporating the jacket, is 532 MHz, which corresponds to 1.95 % at the operating frequency of 27.3 GHz. Fig. 9 displays the radiation pattern in the xy-plane and yz-planes at 27.3 GHz. The original omnidirectional pattern was turned into a directive one with a beamwidth of 37º and 6º in the xy-plane and yz-plane, respectively. As a consequence of significantly reducing the azimuth beamwidth, the realised gain of the antenna array increased by 5.14 db, i.e. from dbi to dbi. Fig. 6. Reflection coefficient of the mechanically reconfigurable SWAA. TABLE I. REFLECTION COEFFICIENT ANALYSIS AS A FUNCTION OF THE JACKET PARAMETERS. n t (mm) S 11 (db) Frequency (GHz) After obtaining a suitable impedance matching, the next step was to analyse the effect of the jacket parameters on the radiation pattern of the antenna. Fig. 7 presents the realized gain as a function of frequency for the values of n and t presented in Table I. The maximum value of realised gain (17.41 dbi), at the operating frequency of the antenna (27.3GHz), was achieved when n = 2 and t = 4 mm. Fig. 8. Reflection coefficient comparison between the original ring-shaped SWAA (without jacket) and the mechanically reconfigurable SWAA with jacket. Fig. 7. Gain analysis of the mechanically reconfigurable SWAA. With the design of the jacket concluded, a numerical investigation of the mechanically reconfigurable SWAA was Fig. 9. Radiation pattern of the mechanically reconfigurable SWAA.

5 Finally, a prototype of the jacket was built using PVC pipe as a mechanical support and aluminium foil to metallise the jacket, as shown in Fig. 10. We envisage, as future work, to fully experimentally characterize the proposed mechanically reconfigurable slotted-waveguide antenna array in terms of reflection coefficient and radiation pattern, as well as implementing it within a real 5G network operating in the mmwave range. Fig. 10. The mechanically reconfigurable SWAA prototype. IV. CONCLUSIONS We have proposed a high-gain mechanically reconfigurable antenna array based on a ring-shaped SWAA for mm-wave applications, including 5G networks. Our technique consists of adding a metallic jacket to properly block part of the radiation from the antenna in order to enhance the main beam gain and enable beam steering in the azimuth plane. In this way, a full 360 scanning range in the azimuth plane is ensured by proper mechanically rotating the metallic jacket. The proposed array provides the following properties: beamwidth of 37º and 6º in the azimuth and elevation planes respectively; bandwidth of 532 MHz (1.95 %) centered at 27.3GHz; gain of dbi at 27.3GHz. The beam steering direction can be very accurately controlled and based on small steps -of only one degree or even less - by using steeper motors to rotate the metallic jacket and, consequently, configure the array radiation pattern. This solution can be considered cost-effective when compared with mm-waves conventional phased array antennas, making it potential for 5G picocells and femtocells. Unlike a conventional phased array, this approach does not suffer from scan loss, beam broadening or SLL degradation. ACKNOWLEDGMENTS This work was partially supported by Finep/Funttel Grant No , under the Radio Communications Reference Centre (CRR) project of the National Institute of Telecommunications (Inatel), Brazil. Authors also thank the financial support from CNPq, CAPES, MCTI and FAPEMIG and technical support from Keysight and ESSS-ANSYS. We would like to acknowledge the support of the University of Surrey 5GIC ( members for this work. REFERENCES [1] T. S. Rappaport, S. Sun, R. Mayzus, H. Zhao, Y. Azar, K. Wang, G. N. Wong, J. K. Schulz, M. Samini and F. Gutierrez, Millimeter Wave Mobile Communication for 5G Cellular: It Will Work!, IEEE Access, vol. 1, pp , May 29, [2] I. Uchendu, James R. Kelly, Combined Parasitic and Phased Array Reconfigurable Antenna, in Loughborough Antennas & Propagation Conference (LAPC), 2015 Loughbotough, [3] J. Costantine, Y. Tawk, S. E. Barbin, C. G. Chritodoulou, Reconfigurable Antennas: Design and Ampplications, Proceedings of the IEE, vol. 3, No. 3, March, [4] I. F. da Costa, Arismar Cerqueira S. Jr, D. H. Spadoti, L. G. da Silva, J. A. J. Ribeiro, S. E. Barbin, Optically Controlled Reconfigurable Antenna Array for mm-wave Applications, IEEE Antennas and Wireless Propagation Letters, in press. [5] R. A. Santos, Arismar Cerqueira S. Jr., Leaky Wave Antenna for Indoor Applications at Millimeter Wave Frequency Band, in 2016 MOMAG, 2016 [In Portuguese]. [6] Robert S. Elliot, "An improved design procedure for small arrays of shunt slots," IEEE Trans. Antennas Propagation, vol. Ap-31, pp , Jan [7] S. Liao et al., "Synthesis, Simulation and Experiment of Unequally Spaced Resonant Slotted-Waveguide Antenna Arrays Based on the Infinite Wavelength Propagation Property of Composite Right/Left- Handed Waveguide," in IEEE Transactions on Antennas and Propagation, vol. 60, no. 7, pp , July [8] S. Liao, P. Chen, P. Wu, K. M. Shum and Q. Xue, "Substrate-Integrated Waveguide-Based 60-GHz Resonant Slotted Waveguide Arrays With Wide Impedance Bandwidth and High Gain," in IEEE Transactions on Antennas and Propagation, vol. 63, no. 7, pp , July [9] Arismar Cerqueira S. Jr., I.F. da Costa, Pinna, S., S. A. de S.Melo, Laghezza, F., Scotti, F., Ghelfi, P., Spadoti, D. H., A. Bogoni. A Novel Dual-polarization and Dual-band Slotted Waveguide Antenna Array for Dual-use Radars. in 10th European Conference on Antennas and Propagation, Davos, [10] Shanjia Xu, Jianhua Min, Song-Tsuen Peng and F. K. Schwering, "A millimetre-wave omnidirectional circular dielectric rod grating antenna," in IEEE Transactions on Antennas and Propagation, vol. 39, no. 7, pp , July, [11] S. Xu et al., A Millimeter-Wave Omnidirectional Dielectric Rod Metallic Grating Antenna, IEEE Trans. Antennas & Propag., vol.44, no.1, pp.74-79, Jan [12] C. Balanis, Antenna theory: analysis and design, John Wiley & Sons, [13] T. Iwasaki et al., A study on Beam-switching Dielectric-rod Antenna using Periodic Metal Plate, IEICE Technical Report, AP , pp.55-60, Feb