Mutual coupling compensation technique using decoupling network for sub-1 GHz multi-antenna mobile terminals

Multiple-input multiple-output (MIMO) technology is a popular technique to improve the channel capacity and it has been extensively employed to enhance high speed data transferring performance in mobile terminal devices. The number of antenna elements on a mobile terminal device with a size of the hand increases with the usage of MIMO technology. The closely spaced antenna elements on a compact size of the ground plane contribute a strong coupling effect among the antennas which leads to performance degradation. This phenomenon commonly occurs especially at frequency band below 1 GHz where the size of the antenna element is theoretically larger, and the gap between antennas is relatively smaller as compared to antenna elements at a higher frequency. This thesis focuses on the implementation of mutual coupling compensation techniques on the different types of antenna elements used on the current mobile devices at low-band Long Term Evolution (LTE) and fifth generation (5G) services, 698 - 960 MHz. Three types of antenna elements were designed as antenna under test for investigation of mutual coupling compensation techniques. The designed antenna elements were monopole antennas, planar inverted-F antennas (PIFAs), and capacitive coupling element (CCE) antennas. PIFA and CCE antenna were designed in a similar dimension for fair comparison purposes. These antennas were designed at the required operational low-band LTE, with at least -6 dB reflection coefficient. The multiple antennas were investigated with three mutual coupling compensation techniques: characteristic mode analysis (CMA), defected ground structure (DGS), and decoupling network (DN). CMA technique is implemented onto all investigated antenna types, whereas the DGS and DN techniques were implemented on PIFA and CCE antennas. The performance of the multiple antennas is analysed in terms of reflection coefficient, mutual coupling, envelope correlation coefficient (ECC), and total efficiency. The implementation of the CMA technique with all antenna types was investigated with five locations which covered the high and low intensity surface current areas on the ground plane. From the results, all investigated antenna types were found to be able to reduce the mutual coupling in the low intensity surface current areas, which are found horizontally at the middle of chassis (locations B and C), whereby the finding is aligned with the theory of CMA. For DGS techniques, the total efficiency for PIFAs and CCE antennas has been improved by 20 % and 35 %, respectively. Meanwhile, the DN technique has improved the total efficiency for PIFAs and CCE antennas by 30 % and 33 %, respectively. On the other hand, the implementation of DGS and DN techniques to compensate the mutual coupling on PIFAs and CCE antennas has offered promising results to reduce the mutual coupling of the highly coupled antennas from only -5 dB up to -26 dB at the centre frequency (i.e., 829 MHz). Further comparisons between DGS and DN techniques have shown that the DN technique exhibits better results than the DGS technique in terms of mutual coupling reduction and total efficiency improvement.