Coupling Cancellation Using Coupling Stubs for MIMO Array Applications

A novel decoupling method using coupling stubs is presented to design a wideband 8 x 8 multiple-input-multiple-output (MIMO) array. The array element consists of two tightly coupled printed inverted-F antennas (PIFAs), which are directly contacted and back-to-back for a compact structure. To decouple this PIFA pair, coupling stubs are elaborately utilized to cancel the different couplings. Detailed equivalent circuit and coupling parameter analyses show that the introduced capacitive coupling from the coupled stubs can well cancel the original inductive coupling in the PIFA pair in the lower frequency band, and the introduced inductive coupling can cancel the original capacitive coupling in the upper-frequency band. Thus, because of the complementary cancellation, the coupling in the original antenna pair is successfully reduced with a wide isolation bandwidth. The 8 x 8 MIMO PIFA pair array was fabricated and measured for the performance investigation. A wide impedance bandwidth of 3.3-4.88 GHz (38.6%) is achieved with the isolation higher than 10.2 dB. In addition, the envelope correlation coefficient of less than 0.092 is obtained for MIMO applications.