  Realizing capacity demands of future wireless communications requires
improved spectral efficiency in the sub-6-GHz frequency bands. This paper
proposes a novel eight-element multiple-input multiple-output (MIMO) antenna
that can be tuned from 1.7 to 6 GHz. The design is based on the antenna cluster
concept, where weighted feeding of multiple antenna elements is used to modify
the operating frequency. This paper extends the theory of the concept to
account for multiple separate clusters, thus enabling it to be used for MIMO.
The proposed antenna achieves over 60% efficiency at frequencies above 3 GHz,
and the system exceeds the ergodic capacity of the ideal 7x7 MIMO in that band.
