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The 5G era for communications has arrived. For both 5G base stations and 5G smartphones, MIMO is a core technology. Without consuming additional spectrum, MIMO allows multiple data streams to be transmitted and received over the same wireless channel, which improves signal paths, increases throughput, and enhances link reliability. As 5G develops, MIMO will become more widespread and larger-scale implementations are likely. The following describes three common methods to improve MIMO isolation.
1. Ground-slot decoupling
Cutting narrow slots in the ground plane between antenna elements is a simple and effective decoupling method. Key design considerations are: (1) the slot position on the ground plane, (2) slot dimensions, and (3) number of slots. A slot with a length about one half wavelength at the antenna operating frequency can increase isolation between elements. two symmetric slots on the ground plane reduce surface-wave and near-field coupling, greatly improving isolation between elements. A slot about one quarter wavelength long can also improve isolation. However, for low-frequency operation the required slot length may become impractically large. Studies show that adding capacitance across the slot can effectively reduce the required slot length.
Introducing ground slots can effectively increase isolation and reduce correlation. The slot on the ground plane behaves like a resonant filter at the operating frequency, suppressing current flow from the excited antenna port to other ports. However, adding slots may degrade antenna impedance matching and alter radiation patterns.
2. Ground-stub decoupling
Ground-stub structures are widely used because they are easy to implement. By extending metal stubs on the ground plane, designers create additional coupling paths whose induced coupling can be adjusted to equal and opposite the original coupling, thereby canceling mutual coupling and improving isolation. Important factors are: (1) the placement of the ground stub and (2) the stub shape. A T-shaped stub is added symmetrically on the ground plane of a pair of F-type MIMO antennas. By tuning the stub length and width so that it resonates at the operating frequency, mutual coupling is reduced and isolation increases. The antenna occupies a 50 x 9 x 0.8 mm volume printed on FR4 (εr = 4.4). To reduce antenna size and widen the impedance bandwidth, coupled feeding is used to provide capacitance to compensate inductive behavior and improve matching. The ground-stub length is tuned to resonate in dual WLAN bands, making it equivalent to a notch element that suppresses near-field coupling to the other antenna port and improves isolation. The effect of stub shape on isolation was studied, and the final design achieved isolation of at least 18 dB across three WLAN bands.
There are many design examples using ground stubs, and stub types depend on the antenna form factor. No single stub shape fits all antennas, so designs lack generality. Additionally, stubs can absorb some radiated energy, which may reduce radiation efficiency.
3. Neutralization line decoupling
Connecting one or more straight or bent microstrip lines between antenna elements to neutralize currents is known as the current neutralization method. Neutralization lines are low-cost, easy to implement, and do not increase antenna footprint, so they have been widely studied. Key issues are: (1) neutralization line placement and (2) neutralization line dimensions. A straight suspended microstrip line connects two PIFA antennas operating at different frequency bands; the overall antenna size is 40 x 100 mm. The neutralization line extracts part of the current from the driven antenna and returns it to the non-driven antenna, producing coupling with opposite phase to the original coupling. This suppresses the effect of the driven element on the non-driven element and improves isolation. The neutralization line length, width, and connection point directly affect the amount of extracted current. Neutralization lines typically provide decoupling only in a single frequency band or narrow band because the effective length needed varies with frequency, which limits their multi-band applicability.
Neutralization-line designs require only connecting a microstrip between elements and do not occupy extra space, which has led to widespread practical use. However, there is no universal rule for defining the neutralization line dimensions and connection points; designers typically perform tuning and joint optimization of the neutralization line and antenna elements with simulation tools, which increases design time and complexity.
