Mobile Antenna Systems for 4G and 5G Applications with User Body Interaction

The cellular communication system has evolved from first generation (1G) analog systems that are only for voice communications, to 4th generation (4G) systems that can support high-throughput data transmission. Recently, the evolution of cellular systems is on the way to 5th generation (5G), which plans to utilize the spectrum over 6 GHz and into millimeter wave (mmWave) frequency range to deploy a wideband mobile network. To cope with the evolutions, various antenna systems have also been developed into mobile terminals in the past decades. The cellular antenna system in a mobile phone has been upgraded from a single antenna to a Multiple-Input Multiple-Output (MIMO) antenna system nowadays, which will be developed into an adaptive array system in future 5G communications. In the thesis, the user body effect on the performance of mobile antennas in a 4G/ Long-Term Evolution (LTE) mobile terminal is discussed first. In order to overcome the degradation of MIMO performance due to the user body effect, a distributed quad-elements MIMO antenna array for mobile terminals is introduced, which mitigates the body effect through an adaptive antenna switching method. Various novel bezel MIMO antennas for the mobile terminal application have also been presented in the thesis. The proposed antennas operate in loop modes, and are robust to the impedance mismatching caused by the user effect. A double-ring shaped bezel structure is also introduced to extend the bandwidth of such kind of designs. The study is further extended to frequency bands at 15 GHz and 28 GHz to investigate the user body effect on mobile antennas and wireless channels for future 5G communications. The body effect on antennas is obtained through 3D far-field measurements of the antenna radiation pattern with a real user. The corresponding impact on characteristics of the wireless channel is investigated with ray-tracing simulations. The results reveal that the user body will cause a strong shadowing loss at 15 GHz and 28 GHz, which will be critical to 5G cellular systems. The electromagnetic field (EMF) exposure of a mobile terminal to the user’s body is also considered in this thesis. For mobile terminals operating below 6 GHz, the specific absorption rate (SAR) is the core metric for EMF exposure compliance tests. The assessment of the simultaneous transmission SAR for MIMO antennas is the major focus of the thesis due to its complicated and time-consuming procedures. The SAR to peak location spacing ratio (SPLSR) is used by Federal Communications Commission (FCC) to determine the exclusion condition of simultaneous transmission SAR assessment, which is an important parameter for the MIMO antenna design in commercial phones. The SPLSR of various 2×2 MIMO antennas are compared in the thesis, to provide a benchmark for MIMO antenna designs in 4G/LTE terminals. For future 5G terminals that operate above 6 GHz, the SAR is not valid anymore as a metric for EMF exposure compliance tests. Instead, the free space power density (PD) is adopted globally as the basic parameter of exposure limits. However, existing regulations on PD are not suitable for mobile terminal applications. Preliminary studies have been presented in this thesis to address the major challenges of PD assessment for the mobile terminal application.