Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Antenna Pattern <s> A textile endfire antenna operating in the 60-GHz band is proposed for wireless body area networks (BANs). The permittivity of the textile substrate has been accurately characterized, and the Yagi-Uda antenna has been fabricated using an ad hoc manufacturing process. Its performance in terms of reflection coefficient, radiation pattern, gain, and efficiency has been studied in free space and on a tissue-equivalent phantom representing the human body. It is shown that the antenna is matched in the 57-64-GHz band. Its measured on-body efficiency and maximum gain equal 48.0% and 11.9 dBi, respectively. To our best knowledge, this is the first textile antenna for on-body wireless communications reported at millimeter waves. <s> BIB001 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Antenna Pattern <s> A pattern-reconfigurable wearable antenna is designed based on a metamaterial structure. By reconfiguring the dispersion curve of the transmission line, the patch antenna can resonate at the zeroth-order mode or the +1 mode, yielding a broadside or an omnidirectional radiation pattern, respectively. The antenna is fabricated with textile material targeting wearable applications. The impedance bandwidth covers the 2.4 GHz Industrial, Scientific, and Medical (ISM) band in both operating states. The antenna is also simulated on a human tissue model, illustrating that the specific absorption rate is well below the European standard threshold. The bending configurations are also discussed for the proposed antenna. <s> BIB002 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Antenna Pattern <s> A promising way of realizing the fifth generation (5G) wireless systems is to operate 5G deployments at higher frequency bands, specifically in the millimeter-wave (mmW) spectrum (30-300 GHz). Access to such spectrum bands will enable future 5G wireless systems to meet the 5G requirements of peak rate greater than 10 Gb/s, and cell edge rate of up to 1 Gb/s. However, the emerging 5G systems will need to coexist with a number of incumbent systems in these bands. This paper provides an extensive study of the co-channel coexistence of 5G in two critical mmW bands, 27.5-28.35 GHz (28 GHz) and 71-76 GHz (70 GHz) bands, where fixed satellite service (FSS) and fixed service (FS), such as wireless backhaul, are the predominant incumbent users. In the 28-GHz study, we show that interference from 5G into the FSS space stations can be kept below the FSS interference protection criterion. We also characterize the minimum separation distance between the FSS earth stations (ESs) and 5G in order to protect the 5G system from interference due to the ESs transmissions. In the 70-GHz study, we show that the 5G-to-FS interference could be a potential issue in certain scenarios, but we introduce techniques to significantly suppress this interference, while maintaining acceptable performance of the 5G systems. For each study, we suggest appropriate deployment strategies for a 5G system based on our results. <s> BIB003 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> B. Antenna Pattern <s> Exposures to radiofrequency (RF) energy above 6 GHz are characterized by shallow energy penetration, typically limited to the skin, but the subsequent increase in skin temperature is largely determined by heat transport in subcutaneous layers. A detailed analysis of the energy reflection, absorption, and power density distribution requires a knowledge of the properties of the skin layers and their variations. We consider an anatomically detailed model consisting of 3 or 4 layers (stratum corneum, viable epidermis plus dermis, subcutaneous fat, and muscle). The distribution of absorbed power in the different tissue layers is estimated based on electrical properties of the tissue layers inferred from measurements of reflected millimeter wavelength energy from skin, and literature data for the electrical properties of fat and muscle. In addition, the thermal response of the model is obtained using Pennes bioheat equation as well as a modified version incorporating blood flow rate-dependent thermal conductivity that provides a good fit to experimentally-found temperature elevations. A greatly simplified 3-layer model (Dermis, Fat, and Muscle) that assumes surface heating in only the skin layer clarifies the contribution of different tissue layers to the increase in surface skin temperature. The model shows that the increase in surface temperature is, under many circumstances, determined by the thermal resistance of subcutaneous tissues even though the RF energy may be deposited almost entirely in the skin layer. The limits of validity of the models and their relevance to setting safety standards are briefly discussed. Bioelectromagnetics. 39:173-189, 2018. © 2018 Wiley Periodicals, Inc. <s> BIB004
We adopt the general antenna pattern equation given by BIB003  where δ denotes the antenna element separation distance, and θ denotes a general angle. Notice that both azimuth and elevation angles are assumed to affect the antenna gain based on Eq. (  where d (m) is the distance between the antennas and f (MHz) represents the operating frequency. 1) Wearable Antenna at 2.4 GHz: Different communication links require different radiation patterns for the wearable operation. When two on-body devices communicate, an omnidirectional pattern seems to meet the requirements. However, when an on-body sensor communicates with an off-body device, a broadside pattern (or patch-like pattern) is more desirable. To support these two operating states, a patch like reconfigurable pattern has been reported BIB002 where the antenna can change its resonance to support both states with the same frequency. The problem with EMF transmissions at 2.4 GHz is more "spread," not just towards the intended recipient. As such, for 2.4 GHz, this paper assumes a microstrip patch antenna which is more of omni-directional in pattern on a certain plane as demonstrated in Fig. 3a . 2) Wearable Antenna at 60 GHz: It is the opposite in communications at 60 GHz. To overcome high attenuation, a communications system in the band usually adopts a "highly directive" radiation pattern. This matches the need in wearables. Commonly, wearable antennas are expected to be low-profile, lightweight, compact, and conformable to the body shape. As such, at 60 GHz, an end-fire pattern has been proposed BIB001 . In such a pattern, the direction of maximum radiation can be set parallel to the body surface. Also, a microstrip-fed Yagi-Uda antenna meets these specifications BIB001 . Fig. 3b shows the antenna pattern of a Yagi-Uda antenna operating at 60 GHz which shows more of a directive radiation pattern compared to the one used at 2.4 GHz. We assumed 16 directors for the 60 GHz pattern as described in Table III. C. SAR Evaluation Fig. 4 represents the human EMF exposure from wearable devices in terms of SAR. The result suggests that the average SAR measured at a certain distance from a wearable device can exceed the existing ICNIRP or FCC guidelines. Fig. 4b shows a zoomed-in display for the range of [0, 5] cm of Fig. 4a . It provides a closer investigation of the minimum safe distance between the wearable devices that are Fig. 4b that for a wearable device operating at 60 GHz, a separation distance of 12 and 15 mm are required according to the current ICNIRP and FCC guidelines, respectively. However, the SAR for operating frequency at 2.4 GHz remains far below the guideline from the very first point. Although one can consider this distance to be very minimal, for a wearable environment, this small distance should also be taken into account for designing wearable devices on the human body. For instance, if a soldier is wearing an on-body device that is mounted on his smart glasses or the VR helmet, its radiation can impact sensitive organs like the eyes or the human brain. The adaptation of (i) higher number of antenna As noted previously, this elevation in SAR can increase the temperature at the surface of the human skin which may have a lethal impact on the human body when dosed in a continued manner or over a long-term period BIB004 .