Wideband Integrated Quad-Element MIMO Antennas Based on Complementary Antenna Pairs for 5G Smartphones

In this article, a wideband and highly-integrated quad-element multiple-input multiple-output (MIMO) antenna is proposed for the first time, to adapt to the size-limited environment in fifth-generation (5G) smartphones. First, the wideband decoupling between two extremely closely-spaced open-slot antennas with face-to-face and back-to-back configurations are investigated. Then, based on the complementary antenna pairs, wideband integrated quad-element MIMO antennas are implemented by the ingenious combination of these antenna pairs. For validation, an 8 ×8 MIMO system, constituted by two sets of integrated quad-antenna configurations, is simulated, fabricated, and measured. Both the simulated and measured results show that the 8×8 MIMO system can provide isolation of better than 10 dB between any two ports and a total antenna efficiency of 52.8%-70.8%/40.5%-75.0% across 3.3-5.0 GHz. Compared with the existing integrated quad-antenna design schemes, the proposed solution can expand the bandwidth from less than 200 to 1700 MHz, covering the entire 5G N77, N78, and N79 bands. In addition to the integrated quad-antenna design, further extension to integrated multiantenna configurations is also discussed by the flexible combination of complementary antenna pairs, which paves the way for future higher-order MIMO system in smartphones.