8‐port multibeam planar UWB‐MIMO antenna with pattern and polarisation diversity
2019; Institution of Engineering and Technology; Volume: 13; Issue: 13 Linguagem: Inglês
10.1049/iet-map.2019.0134
ISSN1751-8733
Autores Tópico(s)Energy Harvesting in Wireless Networks
ResumoIET Microwaves, Antennas & PropagationVolume 13, Issue 13 p. 2297-2302 Research ArticleFree Access 8-port multibeam planar UWB-MIMO antenna with pattern and polarisation diversity Rohit Mathur, Corresponding Author r4rohitmathur@gmail.com orcid.org/0000-0002-3100-2995 Department of Electronics Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, JH, 826004 IndiaSearch for more papers by this authorSantanu Dwari, Department of Electronics Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, JH, 826004 IndiaSearch for more papers by this author Rohit Mathur, Corresponding Author r4rohitmathur@gmail.com orcid.org/0000-0002-3100-2995 Department of Electronics Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, JH, 826004 IndiaSearch for more papers by this authorSantanu Dwari, Department of Electronics Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, JH, 826004 IndiaSearch for more papers by this author First published: 31 July 2019 https://doi.org/10.1049/iet-map.2019.0134Citations: 19AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract In this study, an 8-port multibeam ultrawideband multi-input–multi-output (UWB-MIMO) antenna having pattern and polarisation diversity is investigated for Wireless Personal-Area Network access points. The proposed antenna consists of two different types of slot radiators, each of them provides four directive beams. The novelty of the proposed design is the achievement of a compact 8-port UWB-MIMO antenna using a common ground plane. Eight additional grounded slits are used in this design to enhance isolation. Antenna elements are placed in such a way that two adjacent antennas contribute different patterns. Results show that all the eight antenna elements cover complete UWB (3.1 to 10.6 GHz) with port isolation better than 15 dB. Proposed design offers gain better than 2 dB with envelop correlation coefficient lower than 0.1 from all ports. 1 Introduction Recent wireless consumer electronics devices need higher data rate and throughput in numerous short-range Wireless Personal-Area Network (WPAN) applications. For example Amazon and Google both developed voice-controlled virtual portable assistant (named as Echo Dot and Google Home) to control smart devices for home automation, smart office, smart learning class room, intelligent vehicular network etc. However, the major concern with these kinds of devices is that they use only Bluetooth and Wi-Fi bands which restrict their data rate and a number of simultaneous connections with other devices. This problem is overcome by using license-free ultrawideband (UWB) [[1]] specified by the Federal Communications Commission (FCC) with multi-input–multi-output (MIMO) antenna system. Since FCC specified the strict frequency and power specification of frequency range from 3.1 to 10.6 GHz and power emission limit of 75 nW/MHz, therefore, an increase of number of antenna elements is needed to provide higher data rate with high throughput. In the past ten years, many two-port and four-port UWB multi-input–multi-output (UWB-MIMO) antennas are presented. Most of them concentrate on compactness with reduced mutual coupling [[2]–[6]]. Some of them concentrate on polarisation and pattern diversity [[7]–[14]]. The UWB is achieved using broad band impedance matching of monopole or open slot radiator, whereas improvement of isolation is achieved by different decoupling circuits, for example, ground stub, open λ/4 slot, Y-slot, T-slot etc. Some of the works use orthogonally placed antenna elements for isolation improvement without decoupling network, which also provides polarisation diversity. The several self-complementary designs with counters or orthogonal orientations are also presented in recent literature for two and four-port UWB-MIMO antennas [[10]]. In [[11], [12]] the pattern and polarisation diversity of two-port MIMO antenna is presented by using two diverse antennas placed on the opposed planes of the substrate. In [[13]] pattern and polarisation diversity are achieved using chamfered-edge square patch antenna and two printed dipole antennas for WLAN application. A complex 3D structure having six elements of the three-port ring antenna provides 6 dual polarised broadside beams and 6 omnidirectional beams for pattern and polarisation diversity of 5G application is used in [[14]]. To attain higher throughput 8-port antennas are proposed in [[15]–[17]]. In [[15]] 8-port polarisation diversity MIMO antenna for the 5G band in sub 6 GHz is developed. In [[16], [17]] identical planer monopole antennas and isolated ground planes are used for designing 8-port UWB-MIMO. On the other hand, in [[18]] a 3D eight-element UWB-MIMO antenna is reported. However, not a single work on 8-port UWB-MIMO antennas provides pattern and polarisation diversity simultaneously in a planar configuration. Moreover, 8-port UWB-MIMO antennas [[16]–[18]] use isolated ground planes which make them unfit for MIMO application as suggested in [[19]]. In this paper, an 8-port UWB MIMO antenna which can provide eight distinct beam directions, four directive dual-polarised beams and four dual-polarised broadside beams is proposed. The 8-port MIMO antenna consists of two sets of 4 × 4 UWB-MIMO antennas. This is the first planar 8-port UWB-MIMO antenna with the pattern and polarisation diversity, using a common ground plane. Low correlation between two closely placed antenna elements is achieved using two different feeding methods. 2 Antenna design and analysis The schematics of the proposed 8-port multibeam UWB-MIMO antenna are shown in Fig. 1. The proposed antenna is designed on an FR4 substrate (relative permittivity (ɛr) of 4.4, and a loss tangent of 0.02) having size of 85 × 85 × 0.8 mm3 (1.44λg × 1.44λg × 0.013λg, where λg = λ0 /√ɛe and ɛe = (ɛr + 1)/2 at 3.1 GHz). The dimensions of the structure have been shown in Fig. 1 and same are reported in terms of wavelength in Table 1. The design consists of two different types antennas i.e. open-ended stepped slot antennas (Antenna-A: Ant_1 to Ant_4) and rectangular slot antennas (Antenna-B: Ant_5 to Ant_8). In each type, antennas are placed in an alternative orthogonal arrangement to form a 4 × 4 MIMO. Fig. 1Open in figure viewerPowerPoint Schematic layout of 8-port UWB-MIMO antenna (Antenna-C) (Antenna dimension: L = W = 85, l1 = 2, l2 = 17, l3 = 6, l4 = 9, l5 = 4.9, l6 = 2.5, l7 = 14.5, l8 = 11, l9 = 5.5, l10 = 17, w1 = 2, w2 = 10.4, w3 = 7.5, w4 = 3.6,w5 = 1.6, w6 = 8.5, w7 = 7.9, w8 = 0.5, w9 = 3, w10 = 6, w11 = 16.3, w12 = 29, w13 = 10.8, w14 = 3.2, w15 = 4.4, w16 = 21.3, all dimensions are in millimetres) Table 1. Dimensions of 8-port UWB MIMO antenna in terms of wavelength L l1 l2 l3 l4 l5 1.44λg 0.033λg 0.288λg 0.101λg 0.152λg 0.083λg W l6 l7 l8 l9 l10 1.44λg 0.042λg 0.246λg 0.186λg 0.093λg 0.288λg w1 w2 w3 w4 w5 w6 0.033λg 0.176λg 0.127λg 0.061λg 0.027λg 0.144λg w7 w8 w9 w10 w11 w12 0.134λg 0.008λg 0.050λg 0.101λg 0.276λg 0.492λg w13 w14 w15 w16 — — 0.183λg 0.054λg 0.074λg 0.361λg — — 2.1 Four-element open-ended stepped slot antenna Each element of Antenna-A (Ant_1 to Ant_4) as shown in Fig. 2a is realised by engineering open-ended stepped slots to obtain broadband impedance matching. Each slot is fed by a rectangular patch connected to a 50 Ω microstrip line. The broad opening of slots reduces Q-factor of antenna which results in an increment of band width covering complete 3.1 to 10.6 GHz band. Parameters of feedline and width of the slot (l2 and l3) determine input impedance matching. Band can be tuned by length parameters of slot w1 and w2. The open-ended stepped slots provide directional radiation pattern also. Radiated beams from Ant_1, Ant_2, Ant_3, and Ant_4 are directed in −X, −Y, +X, and +Y directions, respectively. Moreover, polarisations from any two adjacent elements of Antenna-A are orthogonal to each other. So, mutual coupling and correlation between them are very less. Fig. 2Open in figure viewerPowerPoint Design analysis of four element open ended stepped slot antenna (a) Geometry of Antenna-A, (b) Simulated S-Parameters, (c) Simulated surface current distributions, when Ant_1 is excited keeping all other Antenna terminated with match load, (d) 3D radiation patterns of Antenna-A when Ant_1 is excited Fig. 2b shows the simulated S-parameters of Ant_1. The antenna has impedance bandwidth from 2.75 to 11 GHz with port isolation better than 20 dB without any additional isolation improvement mechanism. Fig. 2c shows the surface current distribution over the antenna at three different frequencies when excitation is given at Ant_1 and all other ports are terminated with a matched load. At all the frequencies, most of the current is concentrated around the open-ended stepped slot of Ant_1. It indicates that the signal is not coupled to the other ports and directive radiation is obtained from the excited slot. Fig. 2d shows the directive pattern of a single element at three different frequencies. At all these frequency, patterns are almost similar and maximum radiation remains in the same direction over the band. Owing to symmetry, Ant_2, Ant_3, and Ant_4 have similar S-parameters. 2.2 Four-element rectangular slot antenna On the other hand, each element of Antenna-B shown in Fig. 3a (Ant_5 to Ant_8) consists of rectangular slots [[20]] which are fed by rectangular patches connected to a 50 Ω CPW line. Wide slots provide broadband performance. The input impedance performance of antenna mainly depends on feedline and width of the slot w12. By changing the length of the slot l10, the band can be tuned. The simulated S-parameters of Ant_5 are shown in Fig. 3b. Owing to symmetry, Ant_6, Ant_7, and Ant_8 have similar S-parameters. Fig. 3Open in figure viewerPowerPoint Design analysis of four element rectangular slot antenna (a) Geometry of Antenna-B, (b) Simulated S-Parameters, (c) Simulated surface current distributions, when Ant_5 is excited keeping all other Antenna terminated with match load, (d) 3D radiation patterns of Antenna-B when Ant_5 is excited All the elements of Antenna-B cover the frequency band from 2.6 to 10.6 GHz with isolation better than 15 dB without any additional isolation improvement mechanism because of orthogonal polarisation between two adjacent elements of Antenna-B. The influence of one element of Antenna-B to the others is analysed by the surface current distribution at three different frequencies when Ant_5 is excited and all other ports are terminated with matched loads as shown in Fig. 3c. It shows that only at 6.5 GHz a weak coupling current is present over the adjacent slots which brings |S65| and |S85| near to −15 dB. Fig. 3d shows the broadside bi-directional radiation pattern of antenna at three different frequencies, at the lower frequency it is nearly omnidirectional. 2.3 Eight-element pattern and polarisation diversity UWB-MIMO antenna To develop a UWB-MIMO antenna having multi-directional pattern and polarisation diversity, Antenna-A and Antenna-B are integrated as shown in Fig. 1 (Antenna-C). Antenna-C is a combination of microstrip-fed and CPW-fed slot antennas placed on opposite sides of substrate in alternate configuration. The geometrical orientation of all eight elements provides eight diverse radiation beams. Further, due to close proximity of Antenna-A and Antenna-B elements, a weak coupling will take place in the lower frequency band, which is reduced by creating four open-ended slits at the ground plane. The placement and dimension of slits are optimised by analysing the ground current distribution. Fig. 4 shows the surface current distribution on Antenna-C when either Ant_1 or Ant_5 are excited keeping all other antennas terminated with match load. The coupling current is minimum due to the presence of ground slots at all the three frequencies. Fig. 5 shows the simulated S-Parameters of Antenna-C. Here responses of Ant_1 and Ant_5 are shown. Similar responses by exciting other ports can be obtained due to symmetry. Fig. 5a shows the reflection coefficient of both the antenna, which cover complete UWB from 3 to 10.6 GHz. Fig. 5b shows that port isolations are better than 15 dB. Fig. 4Open in figure viewerPowerPoint Simulated surface current distributions, when either Ant_1 or Ant_5 are excited keeping all other antennas terminated with match load (a) At 3.5 GHz, (b) At 6.5 GHz, (c) At 8.5 GHz Fig. 5Open in figure viewerPowerPoint Simulated S-Parameters of 8-Port UWB-MIMO antenna (a) Reflection coefficient of Ant_1 and Ant_5, (b) Isolation variation Fig. 6 shows the 3D radiation patterns of the proposed antenna from different ports. From Fig. 6 it is verified that Ant_1 to Ant_4 have directive radiation patterns in the four directions covering ±X and ±Y directions, whereas Ant_5 to Ant_8 have bi-directional patterns in the ±Z direction. Thus, all the antenna elements produce diverse beams in different directions which exhibit multibeam patterns and pattern diversity characteristics. Fig. 6Open in figure viewerPowerPoint 3D radiation pattern of each antenna element at 3.5 GHz 3 Result and discussion The prototype of the proposed 8-port antenna is fabricated over FR4 substrate as shown in Fig. 7 and measured using KEYSIGHT N5221A PNA. The measured S-parameters of Ant_1 and Ant_5 are shown in Fig. 8. As a result of symmetry responses of other ports can be obtained from these results. Fig. 8a shows that the measured reflection coefficient is less than −10 dB in the frequency range 3–10.4 GHz. On the other hand, as shown in Fig. 8b the port isolation is below 15 dB in this band. Fig. 7Open in figure viewerPowerPoint Photograph of fabricated antenna Fig. 8Open in figure viewerPowerPoint Measured S-Parameters of 8-Port UWB-MIMO antenna (a) Reflection coefficient of Ant_1 and Ant_5, (b) Isolation variation The 2D radiation pattern of the proposed antenna is measured in an anechoic chamber. The radiation patterns of Ant_1 and Ant_5 are measured in two principal planes at three frequencies 3.5, 6.5 and 8.5 GHz as shown in Fig. 9. Ant_1 has a directional pattern in the XZ-plane, whereas in the YZ-plane it is almost omnidirectional. On the other hand, Ant_5 has a nearly omnidirectional pattern in the XZ-plane and eight-shaped patterns in the YZ-plane. The difference between measured co- and cross-polarisation levels in the direction of maximum radiation from both the ports at 3.5 and 6.5 GHz is more than 20 dB whereas at 8.5 GHz it is better than 15 dB. Fig. 10 shows the peak realised the gain and total efficiency of the Ant_1 and Ant_5. The peak realised a gain of Ant_1 varies from 2 to 4.5 dB with total efficiency greater than 75%, whereas gain of Ant_5 varies from 2 to 6.8 dB with total efficiency better than 70%. Fig. 9Open in figure viewerPowerPoint Measured radiation patterns at (a) 3.5 GHz, (b) 6.5 GHz, (c) 8.5 GHz Fig. 10Open in figure viewerPowerPoint Measured and simulated Gain and efficiency of proposed antenna (a) Gain of Ant_1 and Ant_5, (b) Efficiency of Ant_1 and Ant_5 The MIMO matrices are calculated for Isotropic (XPR = 0 dB), outdoor (XPR = 1 dB) and indoor (XPR = 5 dB) scenario in which outdoor and indoor calculation uses reverberation chamber whereas for Isotropic environment anechoic chamber is also suitable. The performance of the proposed antenna for MIMO operation has been analysed by calculating the envelop correlation coefficient (ECC) and total active reflection coefficient (TARC). Both the MIMO matrices show the antenna performance for Isotropic scenario as the measured parameters are from the anechoic chamber. In this paper, the ECC values of Ant_1 and Ant_5 are computed using simulated 3D radiation patterns following (1) [[21]] (see (1)). The ECC ofthe antenna is evaluated by considering an isotropic statistical model having XPR = 0 dB with a mean elevation angle of polarisation 10° and standard deviation of 15° vertical/horizontal polarised wave in Fig. 11. The ECC of Ant_1 and Ant_5 in all the cases is below 0.1 over the complete band which confirms highly uncorrelated beams further TARC of proposed antenna is calculated using (2) given in [[22]] (1) (2) where is TARC and bi and ai are reflected and incident signals. The scattering matrix S, and these signals are related by the formula (3) Fig. 11Open in figure viewerPowerPoint ECC of proposed antenna for isotropic environment (a) ECC of Ant_1, (b) ECC of Ant_5 Fig. 12 shows the frequency variation of simulated and measured TARC (in dB) of the proposed MIMO antenna. Fig. 12Open in figure viewerPowerPoint Simulated and measured TARC of proposed antenna 4 Conclusion A 8-port UWB-MIMO antenna with pattern and polarisation diversity for WPAN access point application is investigated. The antenna has simple configuration within the size of 85 × 85 mm2. Two different feeding mechanisms are used to put all antenna elements with the same ground plane. Two different types of slots provide a UWB response with distinct radiation patterns. Table 2 shows the comparison of proposed work with other 4-port and 8-port UWB MIMO antennas. It is observed that 4- and 8-port MIMO antennas have polarisation diversity only, whereas the proposed antenna has both polarisation and pattern diversity. Proposed antenna is compact than 8-port antenna of [[15]]. The 8-port antennas reported in [[16]–[18]] which are compact in size than the proposed antenna have the problem of having isolated ground planes. Moreover, the antenna of [[18]] is of non-planar structure. The proposed antenna achieves isolation better than 15 dB due to orthogonal placements of antenna elements and ground plane slits. The measured radiation pattern shows the pattern diversity of proposed antenna. The antenna covers complete UWB with gain better than 2 dB. The ECC value is below 0.1 in all cases for the isotropic environment. Table 2. comparison of proposed antenna with other 4-port and 8-port UWB MIMO antennas Ref. Size, mm3 (in terms of λg) Dielectric constant No. of ports Antenna geometry Diversity type Freq. band, GHz Isolation (in most of the band) Peak gain (measured), dB ECCa (using Far field) [[6]] 60 × 41 × 1 (1.01λg × 0.69λg × 0.01λg) 4.4 4 planar/isolated ground NO 3.1–10.6 ≥20 4–6.5 <0.25 [[8]] 36 × 36 × 0.8 (0.61λg × 0.61λg × 0.01λg) 4.4 4 planar/common ground polarisation 3.1–11.9 ≥15 3.4–6 <0.03 [[9]] 48 × 48 × 0.8 (0.78λg × 0.78λg × 0.01λg) 4.4 4 planar/common ground polarisation 3–11 ≥20 2–4 — [[10]] 50 × 50 × 1 (0.82λg × 0.82λg × 0.01λg) 4.4 4 planar/common ground polarisation 3–12 ≥15 — <0.5 [[15]] 68 × 136 × 6 (0.93λg × 1.86λg × 0.08λg) 4.4 8 planar/common ground polarisation 2.5–2.6 ≥15 ∼2.2–3 <0.2 [[16]] 38 × 90 × 0.76 (0.55λg × 1.30λg × 0.01λg) 3.2 8 planar/isolated ground polarisation 3–15 ≥20 0.5–5 — [[17]] 60 × 93 × 1 (0.98λg × 1.52λg × 0.01λg) 4.4 8 planar/isolated ground polarisation 3–10.6 ≥15 — — [[18]] 50 × 50 × 25 (0.82λg × 0.82λg × 0.41λg) 4.5 8 non-planar/isolated ground polarisation 3–11 ≥20 3–6 <0.5 this work 85 × 85 × 0.8 (1.44λg × 1.44λg × 0.01λg) 4.4 8 planar/common ground pattern and polarisation 3.1–10.6 ≥15 2–4.5/2–6.8 <0.2 a ECC using far field for Isotropic environment only. 5 References [1] Federal Communications Commission (FCC), Revision of Part 15 of the Commission's Rules Regarding Ultra-Wideband Transmission Systems First Rep. and Order, ET Docket 98-153, FCC 02-48, and Adopted: February 2002; Released April 2002 Google Scholar [2]Liu, L., Cheung, S.W., Yuk, T.: ' Compact MIMO antenna for portable devices in UWB applications', IEEE Trans. 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