Dual-Band MIMO Prototype in the Sub-6 GHz Integrated With mm-Wave Arrays: Ensuring Beamforming and Safety Measures

In response to the increasing demands for 4G LTE and 5G smartphone applications, this study presents a compact and integrated prototype designed to cover sub-6 GHz and 5G mm-wave bands with high-frequency ratios. The proposed integrated design incorporates two multiple-input-multiple-output (MIMO) configurations: a
$2\times2$
dual-band planar inverted-F antenna (PIFA) MIMO for sub-6 GHz and an array of eight elements for mm-wave. Proximity placement (2.5 mm separation) of mm-wave elements on smartphone RIM achieves compact array size with optimal beamforming, coverage, and interference prevention. Utilizing a meandered patch and truncated ground, the proposed PIFA achieves a size of 37 mm
$\times7$
mm
$\times0.508$
mm with a Rogers RT/Duroid 5880 substrate. Effective operation is demonstrated in sub-6 GHz bands (5G at 3.8 GHz, LTE band-46 at 5.4 GHz), complying with safety standards (specific absorption rate and power density) set by the Institute of Electrical and Electronics Engineers and the International Commission on Non-Ionizing Radiation Protection. Efficient performance extends to the 5G mm-wave band at 28 GHz, making it a promising choice for modern smartphone applications.