I. Introduction
Next-generation mobile handset designs rely on major advancements in the underlying antenna technologies to implement features of the long-term evolution advanced (LTE-A) and future standards. Multi-input multi-output (MIMO) antenna technology has the potential to greatly improve wireless channel capacity by adopting multiple antennas at both the transmitter and receiver [1]. Low mutual coupling among different antenna elements is required to provide independent wireless channels. Considering the increasing numbers of operating bands and the limited space of mobile terminals, the most significant challenge is realizing MIMO capability within the compact volume of a mobile phone.