A Compact SIW-Based Co-Designed (3.5, 26) GHz MIMO Antenna for Indoor Small-Cell Base Stations

This letter describes a compact, co-designed 16-port multiple-input–multiple-output (MIMO) antenna that operates in both sub-6 GHz and millimeter-wave (mmWave) frequency bands. In the sub-6 GHz range, it features eight interconnected modified octagonal eighth-mode substrate integrated waveguide (SIW) cavities. The TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">010</sub> mode is excited through a U-shape slot inserted on the top surface of SIW cavities. For the mmWave range, eight rectangular quarter-mode SIW cavities are excited at TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">330</sub> mode. They are strategically positioned to create a compact footprint, forming two 8 × 8 MIMO antenna structures. It achieves a measured bandwidth of 30 MHz with a peak gain of 2.29 dBi at the resonance frequency of 3.5 GHz and a bandwidth of 2.48 GHz with a peak gain of 6.73 dBi at 26 GHz. The isolation among the different feeding ports is below 16.3 dB, while the envelope correlation coefficients between various radiated fields are less than 0.14. Key advantages of this proposed MIMO antenna are interband spatial carrier aggregation (CA), low antenna density, and easy tunability across various NR frequency ranges. An example of four 4 × 4 MIMO antennas enabling intraband spatial CA is also investigated. These performances make it suitable for 5G indoor small-cell base stations in densely populated urban environments.