| The fifth-generation(5G)mobile communication technology is being rolled out and coming into commercial application.Up to now,the commercial 5G user equipments are mainly work below 6 GHz(sub-6 GHz),while the 3rd generation partnership project(3GPP)has allocated some millimeter-wave frequency bands to 5G,among which the bands around 28 GHz and 38 GHz might be the firt two to be used by commercial products.Different from the traditional antenna design in user equipment in sub-6 GHz,the antennas designed for the user equipment in millimeter-wave frequency band are expected to have high gain,in order to compensate for the path loss in free space.Meanwhile,the application of millimeter-wave in mobile communication system raises the concern about whether the radio frequency electromagnetic field exposure limits of the international standards could still apply to the exposure from millimeter-wave user equipment.In such background,the following reseaches are done in this paper:1)Following the development tendency of the antenna design in 5G user equipment,patch antenna arrays in millimeter-wave frequency band with different typical sizes are designed.With the human head multilayer model and bioheat transfer equation,the temperature elevation in human tissue are studied with variations of array sizes,beam scanning angles,and distances between antenna and skin.Results show that in close proximity to human head,the antenna with compact structure tends to cause a higher temperature elevation in tissues,and that the beam in boresight elevates the tissue temperature easier than other beams2)With the patch antenna arrays in millimeter-wave frequency,the correlation between the spatial-average incident power density and the temperature elevation in human tissues is studied.It shows that the incident power density averaged over an area about 1 cm2 is almost linearly correlated to the temperature elevation,and the value of the heat factor is close to that of plane wave.3)Considering the scenario that 5G user equipment will likely work in both sub-6 GHz and millimeter-wave frequencies,an expression based on Gaussian distribution to approximate the total exposure ratio(TER)is proposed.Using such an expression,the alignment problem between the specific aborption rate(SAR)measurement sytem and the power densty measurement system can be avoided,and the TER can be estimated accurately.In addition,temperature-based total exposure ratio(TBTER)is also proposed by using heating factors as weights,through which a good linear correlation with temperature elevation in human tissues is promised.4)A fast power density measurement method is proposed to assess the power density in the near-field region of the millimeter-wave antenna in 5G user equipment,based on the equivalent current method.A spherical measurement system is employed in the method,and the measurement of a single antenna within wide band including 28 GHz and 38 GHz only takes 10 minutes.Compared to other method,the proposed method is faster and more flexible.Besides,some important factors that might affect the accuracy of the proposed method are studied.Results show that for a device with a size of our model,the angular resolution of the spherical measurement system should be at least 4° at 28 GHz and 2°at 38 GHz,and the standard deviation of the phase noise in measurement system should be less than 10°. |
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