Integration of Sub-6-GHz and mm-Wave Bands With a Large Frequency Ratio for Future 5G MIMO Applications

The integration of sub-6-GHz and millimeter-wave (mm-wave) bands has become an important issue for future fifth generation (5G) wireless communications owing to their large frequency ratios. This paper proposes a compact-size dual-function antenna operating at 3.5 GHz and the mm-wave band (28 GHz) for 5G mobile applications using a frequency reconfigurability technique. The proposed antenna comprises a microstrip patch linked with a meandered radiating structure through a radio frequency PIN diode to achieve frequency reconfigurability between the two bands. A significant size reduction up to 15.3 $\text {mm} \times 7.2\,\,\text {mm} \times 0.508$ mm for the proposed antenna was achieved using a meandered line structure and truncated ground plane. To enhance the functionality, $8 \times 8$ multiple-input multiple-output (MIMO) with possible long- and short-edge antenna placement configurations were demonstrated. The system exhibited satisfactory MIMO characteristics with wide decoupling −10 dB bandwidths of 7.4% and 4.8% at the low- and high-frequency bands, respectively, without utilizing any external decoupling structure. The simulated results were validated using fabricated prototypes, and good agreement was observed. Additionally, a safety analysis based on the specific absorption rate and power density at the prescribed frequency bands was conducted using a realistic human model, and the results were found to be in accordance with the safety guidelines. Owing to the integration of sub-6-GHz and mm-wave bands in a single compact structure with a large frequency ratio and good MIMO performance, the proposed antenna system is suitable for future 5G mobile handheld devices.