This paper provides a systematic review of the mutual coupling in multiple-input multiple-output (MIMO) systems, including the effects on performances of MIMO systems and various decoupling techniques. The mutual coupling changes the antenna characteristics in an array, and therefore, degrades the system performance of the MIMO system and causes the spectral regrowth. Although the system performance can be partially improved by calibrating out the mutual coupling in the digital domain, it is more effective to use decoupling techniques (from the antenna point) to overcome the mutual coupling effects. Some popular decoupling techniques for MIMO systems (especially for massive MIMO base station antennas) are also presented.

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A Review of Mutual Coupling in MIMO Systems

In this letter, a novel low-profile microstrip-fed compact triple band-notched ultrawideband (UWB) antenna is proposed. Notch bands around the 3.3-3.8-GHz WiMAX and 5.15-5.85-GHz WLAN frequencies are obtained by etching out two elliptic single complementary split-ring resonators (ESCSRRs) of different dimensions from the radiating patch of the antenna. Furthermore, by placing two rectangular split-ring resonators near the feedline-patch junction of the antenna, rejection for the 7.9-8.4-GHz X-band frequencies is achieved. Design guidelines for implementing the notch-bands at the desired frequency regions are provided. The match between the simulated and experimental results suggests that the proposed antenna can be a good candidate for application in UWB communication systems.

A Compact Microstrip-Fed Triple Band-Notched UWB Monopole Antenna

Nowadays synthetic aperture radar (SAR) and multiple-input-multiple-output (MIMO) antenna systems with the capability to radiate waves in more than one pattern and polarization are playing a key role in modern telecommunication and radar systems. This is possible with the use of antenna arrays as they offer advantages of high gain and beamforming capability, which can be utilized for controlling radiation pattern for electromagnetic (EM) interference immunity in wireless systems. However, with the growing demand for compact array antennas, the physical footprint of the arrays needs to be smaller and the consequent of this is severe degradation in the performance of the array resulting from strong mutual-coupling and crosstalk effects between adjacent radiating elements. This review presents a detailed systematic and theoretical study of various mutual-coupling suppression (decoupling) techniques with a strong focus on metamaterial (MTM) and metasurface (MTS) approaches. While the performance of systems employing antenna arrays can be enhanced by calibrating out the interferences digitally, however it is more efficient to apply decoupling techniques at the antenna itself. Previously various simple and cost-effective approaches have been demonstrated to effectively suppress unwanted mutual-coupling in arrays. Such techniques include the use of defected ground structure (DGS), parasitic or slot element, dielectric resonator antenna (DRA), complementary split-ring resonators (CSRR), decoupling networks, P.I.N or varactor diodes, electromagnetic bandgap (EBG) structures, etc. In this review, it is shown that the mutual-coupling reduction methods inspired By MTM and MTS concepts can provide a higher level of isolation between neighbouring radiating elements using easily realizable and cost-effective decoupling configurations that have negligible consequence on the array’s characteristics such as bandwidth, gain and radiation efficiency, and physical footprint.

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A Comprehensive Survey on “Various Decoupling Mechanisms With Focus on Metamaterial and Metasurface Principles Applicable to SAR and MIMO Antenna Systems”

In this communication, a metasurface-based decoupling method (MDM) is proposed to reduce the mutual couplings at two independent bands of two coupled multiple-input-multiple-output (MIMO) antennas. The metasurface superstrate is composed of pairs of non-uniform cut wires with two different lengths. It is compact in size and effective in decoupling two nearby dual-band patch antennas that are strongly coupled in the H-plane with the edge-to-edge spacing of only 0.008 wavelength at low-frequency band (LB). The antenna is fabricated and measured and the results show that the isolation between two dual-band antennas can be improved to more than 25 dB at both 2.5–2.7 GHz and 3.4–3.6 GHz bands, while their reflection coefficients remain to be below −10 dB after the metasurface superstrate is introduced. Moreover, the total efficiency is improved by about 15% in the low band and the envelope correlation coefficient (ECC) between the two antennas is reduced from 0.46 to 0.08 at 2.6 GHz and 0.08 to 0.01 at 3.5 GHz. The proposed method can find plenty of applications in dual-band MIMO and 5G communication systems.

Dual-Band Metasurface-Based Decoupling Method for Two Closely Packed Dual-Band Antennas

In recent years, multiple-input-multiple-output (MIMO) antennas with the ability to radiate waves in more than one pattern and polarization play a great role in modern telecommunication systems. This paper provides a theoretical review of different mutual coupling reduction techniques in MIMO antenna systems. The increase in the mutual coupling can affect the antenna characteristics drastically and therefore degrades the performance of the MIMO systems. It is possible to improve the performance partially by calibrating the mutual coupling in the digital domain. However, the simple and effective approach is to use the techniques, such as defected ground structure, parasitic or slot element, complementary split ring resonator, and decoupling networks which can overcome the mutual coupling effects by means of physical implementation. An extensive discussion on the basis of different mutual coupling reduction techniques, their examples, and comparative study is still rare in the literature. Therefore, in this paper, different MIMO antenna design techniques and all of their mutual coupling reduction techniques through various structures and mechanisms are presented with multiple examples and characteristics comparison.

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Study on Mutual Coupling Reduction Technique for MIMO Antennas

In this paper, we describe the design of a novel planar multiple-input-multiple-output (MIMO) antenna. The basic idea of the design is the development of a canonical two-port antenna that can be replicated and concatenated together to form MIMO antennas with arbitrary even numbers of ports. The design of the canonical element uses compact folded slots for the radiating elements but includes the use of field cancelation to enhance isolation by incorporating a coupling parasitic element. In addition by properly designing the coupling parasitic and the two-port antenna, coupling between canonical elements is also reduced allowing them to be concatenated together. The canonical element size is $27.5 \times 30\;{\rm mm}^2$ operating at 2.6 GHz and it can be packed together with high densities of up to 22 elements per square wavelength. To validate the design, results from 20-port planar printed MIMO antennas are presented operating at 2.6 GHz with a bandwidth of 100 MHz. The 20-port antenna has size of $1.3{\lambda}_{0} \times 0.69{\lambda}_{0}\;{\rm mm}^2$ providing an antenna density of 22 antenna in free space square wavelength ( ${\lambda}_{0}^{2}$ ). Even though the individual antennas are densely packed, all combinations of mutual couplings between ports exhibit better than 10 dB isolation. The antennas are printed on an FR-4 printed circuit board (PCB), which is a low-cost substrate and allows straightforward prototyping.

A Compact Planar Printed MIMO Antenna Design

A novel compact dual-band multiple-input–multiple-output (MIMO) antenna design for IEEE 802.11 applications is proposed. The antenna operates at 4.9–5.725 GHz with four ports and at 2.4–2.5 GHz with two ports and utilizes a dual-band dual-port MIMO antenna to cover the 2.4/5-GHz bands alongside two single-band antennas to cover the 5-GHz band. This allows simultaneous operation at both WLAN frequencies while having only four ports in total. Isolation between ports is better than 12 dB and is achieved with no reconfigurable elements. The overall antenna size is compact, occupying $46\,\times 20\,\times 1.6\:\mathrm{\,\text{mm}^{3}}$ , and is printed on an FR-4 printed circuit board. The proposed antenna is investigated by simulation and measurement, and results include radiation patterns, efficiency, ${S}$ -parameters, signal correlations, and branch power ratios between ports. These show that in typical wireless environments, envelope cross correlations of less than 0.3 between the ports are obtained.

A Dual-Band Multiport MIMO Slot Antenna for WLAN Applications

In this study, a probe fed E-shape fractal patch antenna (EFPA) for heptads' band LTE/WWAN (GSM850/900/1800/1900/UMTS/LTE2300/2500) operation is proposed. Various iterations of this fractal antenna are compared, and an optimized design is presented. Exploitation of E-shape fractal notion in antenna design makes the patch antenna flexible in terms of generating resonances and bandwidth as an iteration order of the fractal is increased. Several properties of the antenna such as impedance bandwidth, radiation patterns, radiation efficiency, electric current distributions on the patch, and gain have been demonstrated numerically and empirically in detail. Experimental results have authorized the design procedure and confirm the gratification of the requirements for multistandard mobile terminal applications.

Design of Simple Multiband Patch Antenna for Mobile Communication Applications Using New E-Shape Fractal

A novel compact four-port reconfigurable multiple-input multiple-output (MIMO) antenna design for IEEE 802.11 applications is proposed. In one configuration, the antenna provides four ports operating from 4.9 to 5.725 GHz with isolation between antennas greater than 14 dB. In the second configuration, it provides a two-port antenna operating at 2.4–2.5 GHz together with another two-port antenna operating at 4.9–5.725 GHz all with isolations greater than 18 dB. The structure consists of four-slot antennas, and two of the slots are made reconfigurable by including MEMS switches in the slots. The overall antenna size is compact and occupies $46\times20\times1.6\;{\rm mm}^{3}$ and is printed on FR-4 printed-circuit-board. The proposed antenna is investigated by simulation and measurement, and results include radiation patterns, S-parameters, and signal correlations and branch power ratio (BPR) between ports. These show that in typical wireless environments’ envelope, cross correlations of less than 0.2 between the ports are obtained. The effect of the RF MEMS switches on the antenna performance is also addressed.

A Port and Frequency Reconfigurable MIMO Slot Antenna for WLAN Applications

In this letter, a compact reconfigurable asymmetric coplanar strip (ACS)-fed monopole antenna with size of 14 $\, \times \,$ 16 mm $^{2}$ that has a complementary reconfigurable performance with capability to operate in the ultrawideband (UWB) frequency from 3.2 to 11 GHz with two stopbands of 3.3–4.2 and 5–6 GHz and triple WLAN frequency bands of 2.4–2.9, 3.6–4.4, and 5–6 GHz is presented. The UWB antenna uses a simple rectangle patch with folded shape slit and stub as rejecting elements, and the mobile antenna uses an arc-shaped stub and inverted L-shaped stub as resonating elements. The performance agility and reconfigurable ability is achieved via a circular rotational motion of the patch. A simple clock stepper motor at the rear of the antenna is used to control the rotational part of the patch and activate the resonating elements. During the simulation, the driving motor effect has been considered. A good agreement is found between the simulated and the measured antenna properties. To the authors' awareness, a feature that the antenna is capable to cover two complementary bands is unique to this letter.

Saber Soltani

Papers

A Compact Planar Printed MIMO Antenna Design

A Dual-Band Multiport MIMO Slot Antenna for WLAN Applications

Design of Simple Multiband Patch Antenna for Mobile Communication Applications Using New E-Shape Fractal

A Port and Frequency Reconfigurable MIMO Slot Antenna for WLAN Applications

Rotatable Dual Band-Notched UWB/Triple-Band WLAN Reconfigurable Antenna