Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Definition of SAR <s> Many different phantoms have been proposed as human head models for experimental estimation of the electromagnetic energy field induced in the human head caused by the use of an RF device. Here, a biotic tissue-equivalent solid phantom is introduced as the human head model realizing the same relative dielectric constant and conductivity as brain tissue and skull layer. This brain-equivalent solid phantom and skull-equivalent phantom make it possible to accomplish highly reliable and precise estimation of specific absorption rate (SAR) in the human head. The phantom models of cube, sphere, and realistic human heads are fabricated. Measurements are performed to estimate the SAR in the human head models exposed to microwave sources by using the thermographic method. <s> BIB001 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Definition of SAR <s> With increasing interest in millimeter-wave wireless communications, investigations on interactions between the human body and millimeter-wave devices are becoming important. This paper gives examples of today's regulatory requirements, and provides an example for a 60 GHz transceiver. Also, the propagation characteristics of millimeter-waves in the presence of the human body are studied, and four models representing different body parts are considered to evaluate thermal effects of millimeter-wave radiation on the body. Simulation results show that about 34% to 42% of the incident power is reflected at the skin surface at 60 GHz. This paper shows that power density is not suitable to determine exposure compliance when millimeter wave devices are used very close to the body. A temperature-based technique for the evaluation of safety compliance is proposed in this paper. <s> BIB002 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Definition of SAR <s> The rapid growth of wireless technology worldwide has prompted interest in the underutilized millimeter-wave band from 30 to 100 GHz. Regulation agencies set limits on the amount of electromagnetic radiation absorbed by users, measuring dosimetric quantities such as specific absorption rate (SAR) to assess compliance. The increased rate of energy deposition in human tissue at high frequencies remains a growing concern as wireless cellular networks begin to incorporate millimeter wave technologies. In this paper, we present a baseband model for SAR-aware wireless communication and a strategy for optimal hybrid precoding in millimeter wave systems with SAR constraints. <s> BIB003 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Definition of SAR <s> Wearable electronics have become the norm over the last few years. Extensive use of wearable wireless devices (WWD) in greater proximity to the body has increased concern about potential biological effects due to the interaction of human tissues with electromagnetic (EM) fields. Human tissues absorb radiofrequency (RF) waves that are capable of affecting energy states at a molecular level, leading to unsafe effects. Remarkably, thermal effect due to the absorption of RF waves is a better biological manifestation than traditional specific absorption rate (SAR) values. In this study, we investigated the application of infrared thermography (IRT) to obtain temperature dynamics and reconstructed average SAR to evaluate the exposure compliance of WWDs. A microstrip-based monopole antenna was used to determine the biological effects of the interaction of EM waves with the body. SAR was obtained using EM field simulations and through reconstruction from thermal measurements with the use of bio-heat equations. The maximum average SAR on the skin was 50 mWkg-1 for the simulations and 54 mWkg-1 from reconstruction after IRT experiments. The maximum temperature change in both cases was observed to be less than 1 °C. The difference between the average SAR obtained through IRT and simulation tools was 8.9% on average. SAR determination from the information acquired using IR temperature dynamics can assess bio-compliance of modern wearable devices for various wireless applications. Bioelectromagnetics. 39:451-459, 2018. © 2018 Wiley Periodicals, Inc. <s> BIB004
The SAR is a quantitative measure of incident energy absorbed per unit of mass and time BIB004 . Intuitively, the SAR quantifies the rate at which the human body absorbs energy from an EMF. Here we review several widely used versions of expressions for SAR. The local SAR value at a point p measured in W/kg  can be expressed as where σ is the conductivity of the material in the unit of siemens per meter (S/m); E is the root-mean-square (RMS) of an electric intensity of the body (V/m); and ρ indicates the density of the material (kg/m 3 ). The SAR can be interpreted with respect to the heat amount as well. Specifically, the SAR can be defined as the ratio of the temperature elevation per unit exposure time. If the heat diffusion is negligibly small during the exposure period, the SAR at an arbitrary point is given by BIB001  where c is the specific heat of the phantom (J/kg/K); T gives the temperature rising at the point of exposure (K); and t is the exposure time length (sec). According to Eq. (2), an abrupt temperature elevation due to EMF exposure can be captured by the definition of SAR. It is noteworthy that the SAR is also inversely proportional to the penetration depth, which suggests a shallower penetration yielding a higher absorption. The SAR at a point on the air-skin boundary can be written as a function of PD(d, φ), which is given by   where R is the reflection coefficient BIB002 ; ρ is the tissue mass density (kg/m 3 ) as defined in Eq. (1); and δ gives the skin penetration depth (m). Notice that PD used in Eq. (3) is formally written as BIB003 PD where P t is a transmit power; G t is a transmit antenna gain; d is the distance (m) from the transmitter. Let us elaborate on the angle φ. It indicates an angle formed between the physical orientation of the antenna and the steered angle of departure BIB003 . The impact of φ becomes more dominant with a directional antenna; as such, if an omni-directional antenna is assumed, this parameter can be ignored. This paper adopts the definition of SAR with φ for the generality of the findings in sequel.