- Open Access
Compact UWB-MIMO antenna with metamaterial FSS decoupling structure
© The Author(s). 2017
Received: 17 November 2016
Accepted: 29 May 2017
Published: 26 June 2017
In this paper, an UWB-MIMO antenna with frequency selective surface (FSS) decoupling structure is proposed. To realize antenna system miniaturization and integration, silicon material is applied as substrate. The proposed antenna becomes a very compact construction with dimension of 38.2 × 26.6 × 0.4 mm3. The proposed broadband FSS unit consists of four split rectangles and one I-shaped strip. The effective permittivity or effective permeability of FSS unit is negative, so the FSS is a metamaterial structure and has a broadband frequency band gap in the entire UWB range. Six FSS units are positioned in the middle of antenna backside, like band-stop filter to reduce the coupling between the antennas placed side-by-side. Compared to UWB-MIMO antenna without FSS, the proposed array not only keeps each antenna performance but also decreases 7.2 dB coupling. The UWB-MIMO with FSS structure provides an overall isolation of more than 16 dB in the UWB spectrum, so it is suitable for portable UWB-MIMO system applications.
The ultra-wideband (UWB) system is a radio engineering technology with frequency from 3.1–10.6 GHz , which has several advantages including very large bandwidth, low power consumption, high date rate, high time resolution, resistance to interference, co-existence with narrowband systems, and so on. Such advantages enable UWB technology widely applied in communication, radar, imaging, and positioning [2–5]. To improve transmission rate and communication reliability of UWB system, multiple-input-multiple-output (MIMO) technology can be joined to become UWB-MIMO system. The combinative system needs multiple antennas coexisting in the finite space of transmitters and receivers. The individual antenna not only has broad impedance matching characteristic over the entire spectrum but also has better isolation from adjacent antenna. The coupling influence between antennas is more serious as the increase of the antenna number, so the effective decoupling method is a key technology for UWB-MIMO system. The straightforward decoupling way is to extend separation distance of antennas, but the size of each component is strictly controlled for miniaturization UWB mobile terminals. The correct scheme is to design special structure to decrease coupling without sacrificing transceiver space. For plane monopole MIMO antennas, the common decoupling structures are parasitic elements and defected grounds. The detailed structures are diversified such as T-shaped element, Y-shaped element, or combination with several long and short strips [6–8]. Another approach for designing UWB-MIMO antennas is to use slot antennas with orthogonal feeding to achieve polarization diversity and pattern diversity [9–12]. These layouts are relatively larger than the above decoupling antenna structures. Frequency selective surface (FSS) is a period electromagnetism material with frequency band gap performance. FSS structures can selectively determine electromagnetic waves to pass or prevent within specified frequency ranges.
In this paper, a UWB-MIMO structure of antenna is presented. The broadband FSS cell is composed of electric resonator and magnetic resonator with the band gap characteristic as same as metamaterial. This UWB-MIMO antenna with FSS structure obtains more than 16 dB isolation between antennas in the UWB frequency band. The rest of this paper is as follows: Section 2 presents the antenna configuration, Section 3 discusses the performances of metamaterial FSS unit, Section 4 analyzes the results of UWB-MIMO antenna, and finally, the paper is concluded in Section 5.
2 Antenna structure design
According to UWB technology in small mobile devices, the antenna is designed on planar microstrip transmission line with coplanar waveguide (CPW) feeder. The 400 μm thickness silicon wafer is chosen as antenna substrate. Because the relative dielectric constant of silicon is 11.9, the size of MIMO antenna can be shrunk. Then, silicon wafer is the material of most of integrated circuits, so antennas with silicon substrate are easily integrated with functional circuits to become a complete on-chip system.
Dimensions of the UWB-MIMO antenna
where ε 0 is the permittivity of vacuum, ε r is relative permittivity of silicon, h 1 is the height of air cavity and h is the thickness of silicon wafer. The vertical air cavities enable to decrease relative dielectric constant, solve surface wave problem, and improve MIMO antenna performances.
3 Decoupling structure design
When k is a real number, electromagnetic waves can be propagated in the medium. While ε or μ is a negative value, the propagation constant k becomes an imaginary number. In this condition, electromagnetic waves cannot be propagated and the band gap appears. Therefore, when electromagnetic waves are incident on the surface of metamaterials, total reflection phenomenon will occur.
The UWB-MIMO antenna is proposed with novel decoupling structures, which are the front defected ground structure and the rear FSS structure. Because of the CPW feeder of antenna, the middle grounds are connected together with mutual effect in high-frequency band of UWB. The defected ground transformation reduces coupling of connected ground, and the center short-circuited strip plays a role of reflector between two antennas. Broadband FSS structure is positioned in the middle of cavities. As compact antenna dimension, FSS should be placed on the back side to lower the direct impact on the front radiation patches.
4 Results and discussions
This paper proposes a novel MIMO antenna based on silicon substrate for UWB communication systems. To achieve high isolation between antennas, six metamaterial FSS units are employed in the middle of array. Finally, the isolation performance is less than −16 dB over the entire UWB frequency band. More importantly, the designed MIMO antenna becomes a very compact construction especially suitable for mobile terminals, like notebook computer or mobile, where antennas need to satisfy the demands of low profile, small dimensions, and integration with other components.
This work is supported by National Natural Science Foundation general projects, China (No.61471056), the Youth Learning Scholar Supporting Program of Colleges and Universities of Heilongjiang Province, China (No.1254G051), and the Natural Science Foundation of Heilongjiang Province, China (No. F201322).
This work is supported by National Natural Science Foundation general projects, China (No. 61471056), the Youth Learning Scholar Supporting Program of Colleges and Universities of Heilongjiang Province, China (No.1254G051), and the Natural Science Foundation of Heilongjiang Province, China (No. F201322).
XZ proposed the main idea and completed the antenna construction design. XY assisted the theory research. QS and BL assisted the simulation and analysis. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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