Study of Decoupling between Closely Spaced Microstrip MIMO antennas using Microstrip Resonator

This paper presents a novel decoupling technique between two closely spaced MIMO antennas whose edge to edge spacing is0.03λ (1.975mm).A microstrip resonator is used in between the antennas that act as a decoupled device. The MIMO plays an important role in the current communication system due to its enhanced data transferring rate. However, the closely spaced MIMO antennas have a major disadvantage of high mutual coupling (MC). MC affects the entire characteristics of MIMO antennas and as a result, the performance of the antenna degrades. To overcome this problem, we have introduced a microstrip resonator that acts as an electrical wall between the MIMO antennas. The antenna has been simulated by using an IE3D EM simulator and a suppression of 48dB MC is achieved at a resonant frequency of 4.85 GHz. The MIMO antennas have been fabricated and the results are compared with the simulated ones. This antenna can be used in wireless communication, WLAN and satellite communication.


Introduction
Microstrip antennas are applicable to spacecraft, aircraft, and missiles as these antennas are light weight, low fabrication cost, exible, and also support both linear and circular polarization [1]. Microstrip array and MIMO structures can easily be constructed therefore; it grows interest on the researchers to design various array and MIMO con gurations for achieving better performances such as directivity and data throughput. In our attempt, we have designed a two-element MIMO structure and the MC between the closely spaced MIMO antenna elements has been optimized by introducing a microstrip resonator between the antenna elements. It has been seen that the MC is increased with decreasing the separation between antenna elements of the array [2]. Therefore, to design a compact array/MIMO structure, it is required to maintain low MC to achieve better array/MIMO performances. Few works on MC have already been reported in the open literature where researchers used Defected Ground Structure (DGS) [3,4] and Electromagnetic Band Gap (EBG) [5] structure to tackle mutual coupling. These methods provide high back radiation and the results are the radiation loss in the principal plane [6]. Some other techniques have also been introduced to reduced mutual coupling, such as meander line resonators [7], microstrip Ushaped resonator [8], folded split-ring resonators [9], metamaterial inspired superstrate [10], slottedcomplementary split-ring resonators [11], microstrip I-shaped resonator [12] etc. These structures also suffer from some disadvantages. In [7] Jeet Ghosh et al. proposed a meander line resonator in between two microstrip patch antennas. Here, the authors have achieved an 8dB reduction of MC at. Saeed Farsi et al. in [8] have used a simple microstrip U-section in between the two antennas. In this design, the center to center distance between the antenna elements is kept 0.6λ and this decoupling structure reduces the MC by 10 dB at a resonance frequency of 2.4 GHz. In [9]authors proposed folded split-ring resonators in between two closely spaced antenna elements (edge to edge spacing 0.039λ).In this design, MC is reduced by 30dB at a resonant frequency of 5.2GHz.Liming Si et al. have used metamaterial-inspired superstrate [10] as a decoupling structure and 29dB reduction of MC has been achieved in this design. In [11], MC of 10dB at 5.0 GHz has been suppressed at the cost of a complicated metamaterial structure used in between two antennas. A simple I-shaped resonator structure composed of two E-shaped microstrip antennas with centre to centre spacing of 0.45 λ is presented in [12].Here, 30dB suppression of MC has been achieved. In spite of few disadvantages, some good papers [13][14][15][16][17] in this line are referred to comparison (Table-1). In our work, we have proposed two-element MIMO antennas located extremely closed to each other (1.975mm or 0.03λ, edge to edge distance) with a decoupling structure loaded with few rectangular slots is placed in between the antenna elements. In this attempt, suppression of MC of 48 dB is achieved at a resonant frequency of 4.85GHz. The proposed design is simulated using the method of moment (MOM) based IE3D simulator and the electrical characteristics are thoroughly investigated.

Antenna Design And Result
To investigate the novel decoupling method two identical rectangular patch antennas with inset line feed technique has been considered. The single element rectangular patch antenna with dimensions is shown in Figure- A return loss (S 11 ) of -30 dB at resonance (4.85 GHz) is observed from Figure-1 . Figure-2(a) shows the schematic of the proposed antennas with inset line feed where edge to edge spacing of 0.03λ and center to center distance between the two antenna elements is 0.25λ. Table-1 shows the design parameter (mm) of the proposed antenna. As the two antenna elements are in close proximity the MC between the antenna elements will degrade the MIMO antenna characteristics including radiation patterns in the principal plane as the surface current and near elds take a major role for MC between the patches. The simple microstrip resonator of length 21.8mm (0.35λ) loaded with rectangular slots is introduced in this design to reduce MC as shown in Figure-2 Figure-3 illustrates the Sparameter characteristics with and without resonator structure. From Figure-3, it is observed that the MC is suppressed by 48 dB when the resonator is inserted between the patches. In this investigation, MC of 7dB without resonator and 54dBwith resonator at resonance is observed. From Figure-4 Figure-7 shows a study of simulated co-polar and cross-polar radiation with and without resonator From Figure-7, we observed that there is no signi cant difference between co-polar radiations in both cases (with and without resonator). It is also seen that 7 dBi cross-polar radiations is suppressed when a resonator is introduced in between the two antenna elements of the MIMO structure. A parametric study of S 21 is done ( Figure-

Antenna Fabric And Measurements
The prototype antennas with and without resonators are fabricated using FR4 substrate as shown in the Figure-9.
The return loss characteristic of the prototype antenna with resonator has been measured and the results are illustrated in the Figure-10. From Figure-10, it is seen that there is a little difference between simulated and measured return loss characteristic. This may be due to the little imperfection of fabrication process of the antenna.
Measured and simulated S-parameters result with and without resonator are shown in the Figure-11.
From Figure-11, it is observed that there is a slight mismatch between simulated and measured S21 characteristics. This may be due to the measurement and fabrication error of the antenna. The measured S21 values show that there is a suppression of MC of 42 dB which is noticeable suppression of MC.        Measured and simulated S-parameters results with and without a resonator Antenna e ciency with resonator Measured radiation patterns with and without a resonator