Abstract-In the paper, a concept and design of a compact MIMO antenna is presented. In our approach, an improved space filling is obtained by constraining an individual antenna layout and optimizing it for minimum size. The MIMO design is constructed by composing the two of previously designed structures. Because the design problem is heavily constrained both in terms of geometry and electrical performance objectives, surrogate-based optimization techniques are used to accomplish the design process in reasonable timeframe. The final MIMO design features a small size of only 565 mm 2 and exhibits good isolation in the entire UWB band.Index Terms-UWB antennas, MIMO antennas, computeraided design, EM-driven design, topology optimization, surrogate-based optimization.