Due to the great demand for high-speed wireless communication, Multi Input Multi Output (MIMO) antenna systems have been attracted rapidly increased attention. Therefore, the interaction between human body and MIMO antenna becomes an important issue, which will be studied in the present thesis. This thesis mainly focuses on the specific absorption rate (SAR) of the LTE MIMO antenna in mobile phone. It is different from the SISO antenna, and as the MIMO antenna has more operation modes and functions (MIMO, SISO, beam forming, etc.), more parameters need to be investigated for the SAR of the antenna. In this thesis, four designs of dual elements MIMO antenna (dual semi ground free PIFA, co-located antenna, dual OG PIFA in parallel position and dual OG PIFA in orthogonal position) are studied under four typical LTE frequency points (0.75GHz, 0.85GHz 1.9Ghz and 2.1/2.6GHz), with the effects of different chassis lengths (90mm, 110mm, 130mm and 150mm), the port’s states (terminated with 50ohm, open or short) and phase difference (0 degree to 360 degree). The SAR, when dual elements operate simultaneously, is also studied, which is evaluated by the SAR to PEAK Location Spacing Ratio (SPLSR). The simulations are run on both a SAM head phantom and a flat phantom by CST 2011, and measurements on flat phantom are carried out with iSAR and Dasy to verify the accuracy of our simulations. Besides SAR, the body loss of MIMO and SISO LTE antennas (MIMO: dual semi ground free PIFA and co-located Antenna; SISO: OG PIFA on top and OG PIFA at the bottom) in mobile phones are also studied in this thesis. The body loss and multiplex efficiency (MUX) with head and hand (H+H) in an increased cell phone case length (110mm, 130mm and 150mm) are investigated and discussed. Furthermore, some prominent problems in body loss study (body loss reduction, right and left hand problem, etc.) are also discussed in this thesis.

In the thesis, the user body effect on antennas in a mobile terminal is discussed. In order to overcome the degradation of Multiple-Input Multiple-Output (MIMO) performance due to the user body effect, a quad-elements MIMO antenna array which can mitigate the body effect through an adaptive antenna switching method is introduced for 4G mobile terminals. In addition, various bezel MIMO antennas that are robust to the impedance mismatching caused by the user effect have also been presented. The study of user body effect is later extended to frequency bands at 15 GHz and 28 GHz for future 5G communication systems. The results reveal that a human body will cause a significant shadowing loss, which will be a critical loss in 5G cellular networks. The electromagnetic field (EMF) exposure of a mobile terminal is also studied in this thesis. Below 6 GHz, the simultaneous transmission specific absorption rate (SAR) for MIMO antennas is the primary concern due to its complicated assessment procedures. Above 6 GHz, the free space power density is adopted as the basic parameter of exposure limits globally, and preliminary studies have been presented to address major challenges in EMF exposure assessment for 5G mobile terminals. / <p>QC 20171005</p>