**5. User influence**

As a special situation for the mobile antenna, the effect of the human tissue on the antenna performance should be considered when designing the mobile antenna. Also, the electromagnetic field (EMF) exposure to the mobile antenna for the human tissue should also be studied for safety considerations. This is because the mobile antenna is usually close to the human head, body, or hands, as is used in practical situations. In this section, the mutual effect of the mm-wave array and the human tissue in the open literature is summarized.

The human body is electronically large at the mm-wave frequency band. To evaluate the mutual effect of the human tissue and the mm-wave array effectively and accurately, [92] developed the numerical and physical phantoms of a human body for evaluation of mobile antennas at 28 GHz. Thus, the ability to design antennas under practical operational conditions involving body effects is achieved. As for the EMF exposure to the mm-wave array, [93] compares the power density of a single antenna element, a four-element linear array, and an eight-element linear array at the near field region and the far field region. Zhao et al. [93] finds that, at the near field region, the power density is extremely high and it can be reduced as the number of array elements increases. At the far field region, the power density increases as the array elements increase.

As for the effect of the human on the mm-wave array, [76] shows that, for an eight-element mm-wave array located alongside the side edge of the mobile phone, the human hand results in a gain reduction of about 7.5 dB. However, for a fourelement mm-wave array, if the human hand covers the mm-wave array, the loss from hand blockage on the antenna gains can be up to 20 to 25 dB [94]. To reduce the effect from the human hand, the mm-wave array should be deployed away from the human hand. Ojaroudiparchin et al. [95] illustrates that, if the mm-wave array is not close to the human hand, the gain loss from the human hand can be reduced to about 1.5 dB. Therefore, to achieve a good performance, multiple mm-wave arrays should be deployed in the mobile phone at different positions. In [96], it is concluded that, in the talk mode, the mm-wave array should be placed at the top of the mobile phone (close to the index finger). Also, it the talk mode, the user hand shadowing can be significantly reduced by placing the mm-wave array at the bottom of the chassis (close to the palm). In the data mode, the mm-wave array achieves less gain loss when deployed at the top of the mobile phone.

For the human body shadowing, [97] illustrates that the shadowing by the user's body might decrease the gain about 20–30 dB if the mm-wave array is close to the user. Zhao et al. [98] also observes a strong shadowing effect from the human body in the mm-wave band, which is around 20–25 dB at 15 GHz. Zhao et al. [99] finds that the equivalent isotropic radiated power (EIRP) values at cumulative distribution function (CDF) of 50% drop about 5–10 dB compared to the case that without the user body. To reduce the shadowing from the human body, the effect of the displacement of the mmwave array on the shadowing from the human body is studied [100]. Syrytsin et al. [100] finds that the corner positions of the mobile phone achieve the best performance in terms of spatial coverage. Syrytsin et al. [101] compares the coverage efficiency and user shadowing from the mm-wave phased array and mm-wave switch diversity array and finds that the mm-wave phased array has superior performance. Also, in [102], it is found that, for the difference between the user and free-space cases, the circularly polarized array coverage efficiency is relatively less sensitive to user effects.
