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

A compact multiple-input-multiple-output (MIMO) antenna is presented for ultrawideband (UWB) applications. The antenna consists of two open L-shaped slot (LS) antenna elements and a narrow slot on the ground plane. The antenna elements are placed perpendicularly to each other to obtain high isolation, and the narrow slot is added to reduce the mutual coupling of antenna elements in the low frequency band (3-4.5 GHz). The proposed MIMO antenna has a compact size of 32 ×32 mm
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, and the antenna prototype is fabricated and measured. The measured results show that the proposed antenna design achieves an impedance bandwidth of larger than 3.1-10.6 GHz, low mutual coupling of less than 15 dB, and a low envelope correlation coefficient of better than 0.02 across the frequency band, which are suitable for portable UWB applications.

Compact Printed MIMO Antenna for UWB Applications

A multiple-input-multiple-output (MIMO) band-notched antenna with a compact size of only $22 \times 36\;\mathrm{mm}^2 $ is proposed for portable ultrawideband (UWB) applications. The antenna consists of two square monopole-antenna elements, a T-shaped ground stub, a vertical slot cut on the T-shaped ground stub to reduce mutual coupling, and two strips on the ground plane to create a notched frequency band. Simulation and measurement are used to study the antenna performance in terms of impedance matching, isolation between the two input ports, radiation pattern, efficiency, peak gain, and envelope correlation coefficient. Results show that the antenna can operate from 3.1 to more than 11 GHz with a notched band in 5.15–5.85 GHz. In the center notched frequency, the efficiency drops to 7%, indicating a good interference suppression performance. The mutual coupling is less than $- 15\;\mathrm{dB}$ and the envelope correlation coefficient is less than 0.1. The performances of the MIMO antenna when installed on a PCB with a standard size, with an USB connector and device housing, are also studied. Results show that the proposed MIMO antenna is a good candidate for portable UWB applications.

Compact MIMO Antenna for Portable UWB Applications With Band-Notched Characteristic

A compact four-element multiple-input–multiple-output (MIMO) antenna for portable wireless ultrawideband (UWB) applications is presented in this letter. The proposed UWB MIMO antenna has the compact size of $ 42 \times 25 ~\hbox{mm}^2$ . The microstrip-fed stepped-slot antenna acts as a single UWB antenna element. The UWB antenna covers the wide frequency band from 3.1 to 12 GHz. High isolation ( $ \geq 22~\hbox{dB}$ ) is achieved between the UWB antenna elements without any decoupling network. It is due to inherent directional radiation properties of slot antennas (SAs) and their asymmetrical placements. The measured impedance bandwidth of UWB MIMO antenna is from 3.2 to 12 GHz with the isolation better than 22 dB over the entire UWB spectrum. The envelope correlation coefficient (ECC) as well as channel capacity loss (CCL) are investigated, and they are within their acceptable limits.

Compact MIMO Slot Antenna for UWB Applications

In this letter, a structure of a small ultra-wideband (UWB) monopole antenna, its design optimization procedure as well as experimental validation are presented. According to our approach, antenna compactness is achieved by means of a meander line for current path enlargement as well as the two parameterized slits providing additional degrees of freedom that help to ensure good impedance matching. For the sake of reliability, the antenna design process (simultaneous adjustment of multiple geometry parameters) is carried out using high-fidelity EM analyses. Surrogate-based optimization involving an auxiliary coarse-discretization EM model it utilized to accomplish the design in practical timeframe. Penalty function approach allows us to reduce the antenna footprint (to only $15.8 \times 22~\hbox{mm}^2$ ) while maintaining acceptable reflection in the UWB frequency range. Experimental validation of the design is also provided.

Structure and Computationally Efficient Simulation-Driven Design of Compact UWB Monopole Antenna

A compact dual band-notched ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna with high isolation is designed on a FR4 substrate (27 × 30 × 0.8 mm3). To improve the input impedance matching and increase the isolation for the frequencies ≥ 4.0 GHz, the two antenna elements with compact size of 5.5 × 11 mm2 are connected to the two protruded ground parts, respectively. A 1/3 λ rectangular metal strip producing a 1.0 λ loop path with the corresponding antenna element is used to obtain the notched frequency from 5.15 to 5.85 GHz. For the rejected band of 3.30-3.70 GHz, a 1/4 λ open slot is etched into the radiator. Moreover, the two protruded ground parts are connected by a compact metal strip to reduce the mutual coupling for the band of 3.0-4.0 GHz. The simulated and measured results show a bandwidth with |S11| ≤ -10 dB, |S21| ≤ -20 dB and frequency ranged from 3.0 to 11.0 GHz excluding the two rejected bands, is achieved, and all the measured and calculated results show the proposed UWB MIMO antenna is a good candidate for UWB MIMO systems.

Compact Dual Band-Notched UWB MIMO Antenna With High Isolation

A new broadband multiple-input multiple-output (MIMO) antenna with good isolation and compact size is proposed. The proposed antenna consists of two G-shaped elements in the upper layer and two inverted L protrude branches and a T slot etched in ground which is used to reduce the mutual coupling. This planar antenna has a bandwidth of 100% with jS11j • i10dB from 2.26 to 6.78GHz. The value of isolation between the two antenna elements is more than 22.5dB in the whole band. The experimental results verify the simulations.

https://www.jpier.org/pierc/pierc41/13.13042104.pdf

Xing-Xing Xia

Papers

Compact Dual Band-Notched UWB MIMO Antenna With High Isolation

Design of a Compact Wideband MIMO Antenna for Mobile Terminals