Self-Isolation in MIMO Array Using Mode Diversity

A new isolation technique is proposed for multiple-input multiple-output (MIMO) microstrip patch antenna (MPA) arrays utilizing mode diversity. Typically, similar modes are excited in each element of the MIMO array. However, the proposed MIMO array uses two elements of different sizes excited in distinct modes at the same frequency. It is found that the TM01 mode excited in one MPA, a hybrid mode (TM $_{03} +$ TM21) is induced in the coupled MPA, while exciting the TM03 mode in another patch induces a quasi-TM11 mode in the coupled MPA. Both induced modes exhibit weak fields at the feeding points, reducing the mutual coupling between the two patch antennas without requiring extra decoupling structure. Additionally, the induced modes create a null in the broadside direction, reducing cross-polarization (x-pol) levels. An isolation level over 50 dB is achieved in $1\times 2$ MIMO array at 5.8 GHz. The design is adaptable to larger array. Therefore, to validate the design technique, a $1\times 4$ array is simulated, fabricated and measured, showing 30–45 dB isolation among the ports in the passband. Moreover, the design features an x-pol around -40 dB in broadside direction in both E- and H-planes, ensuring effective polarization diversity. The technique is applied to a MIMO dielectric resonator antenna (DRA) array, demonstrating excellent isolation and radiation pattern performance. Furthermore, the technique is versatile and can be applied to various MIMO configurations. Therefore, simulations have been performed for $2\times 4$ MPA array, validating the effectiveness of isolation in both the E- and H-planes. This approach also optimizes space utilization, allowing for the inclusion of additional patch elements to form hybrid-coupled MIMO MPAs within confined spaces.