Analysis and Estimation of MIMO-SAR for Multi-antenna Mobile Handsets

Measuring multiple-input multiple-output (MIMO) specific absorption rate (SAR) is an important part of mobile handset evaluation, following the introduction of uplink MIMO transmission schemes in long-term evolution advanced systems. However, the measurement of MIMO-SAR is complex and time consuming. In this communication, six unique dual-antenna mobile handsets, including both typical and novel designs, are analyzed to determine the dependence of MIMO-SAR on antenna configuration and correlation. It is found that, for certain antenna configurations, the location of the maximum MIMO-SAR and the corresponding relative phase between the ports can be predicted. This can be applied to drastically reduce measurement time. In addition, dual antennas with low envelope correlation coefficients in the radiation patterns also offer near-orthogonal electric near fields. This leads to smaller MIMO-SAR than stand-alone SAR (S-SAR) over all relative phases, making MIMO-SAR measurement unnecessary. The results also provide guidelines for designing multi-antenna handsets with low SARs. For verification, the S-SAR and MIMO-SAR at a relative phase of 0° were measured for several prototypes. The measured SAR distributions showed good agreement with the simulated ones.