Pixel-Metastructure-Assisted Subarray Decoupling for Compact MIMO Antenna Designs

This paper presents a method for constructing compact multiple-input multiple-output (MIMO) antennas based on subarray partition and decoupling. An efficient two-step approach is proposed to transform the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i>-dimensional scattering matrix into a zero matrix. First, the intra-subarray coupling of two co-located-fed antenna elements is counteracted using a section of coupled transmission line. Second, the topology-optimized pixel metastructures, with flexibly controllable network response, are introduced between subarrays for inter-subarray decoupling. By partitioning the high-dimensional scattering matrix into sub-blocks describing intra- and inter-subarray couplings, the multi-element array decoupling problem can be decomposed into smaller-scale sub-problems and solved step by step. To verify the proposed design method, a compact 4-element planar MIMO array with an area of merely 0.603λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> × 0.414λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> (at the lowest frequency of the 15-dB isolation band) is prototyped and experimentally characterized, which offers a 15-dB isolation bandwidth of 11.0% and total efficiency up to 92%. Finally, extension of the design to a six-port MIMO array is demonstrated.