Specific Absorption Rate Compliance Research Articles

A Precoding and Power Control Scheme for SAR Reduction in Indoor MIMO Multicarrier Systems

In this work, a novel strategy for lowering the uplink user exposure index (EI) is described for an indoor narrowband use-case. This metric measures the long term exposure to electromagnetic radiation a user receives from a device operating in the uplink from the user terminal to the access point. The specific type of device chosen for the exposition of this novel method was a laptop computer and results presented herein specifically target an aspect of this metric called the specific absorption rate (SAR). It is firstly shown that SAR, in the context of a laptop, may be modeled in a similar fashion to the more familiar smartphone analyses that appear in literature. Secondly, an algorithm comprising a mixture of precoding and power control is proposed for usage in the uplink where it is seen to reduce uplink user EI. This proposed approach can provide a reduction in the long-term exposure of the user or be used as a means to increase the transmit power of the device while maintaining SAR compliance. It is shown that if quality-of-service (QoS) is maintained, the proposed approach can achieve a median reduction in SAR of 50 %, which in turn is seen to lower the EI by 30 %. Furthermore, a 60 % median reduction in SAR may also be possible if a minor degree of decrease in QoS is tolerated, which in turn is seen to lead to a 50 % reduction in the EI.

Determining Electromagnetic Exposure Compliance of Multi-Antenna Devices in Linear Time

Electromagnetic exposure metrics such as specific absorption rate (SAR) are used to assess the user safety of portable wireless devices under all transmitting conditions. When there are multiple transmitter chains operating at a single frequency, the process of determining compliance is complicated by the fact that testing needs to cover all possible combinations of transmissions from all antennas. For $N$ antennas, the number of possible distinct transmitted signals can grow quickly with $N$ . We propose a compliance test whose measurement time burden is linear in $N$ , independently of the number or type of transmitted signals. For $N\geq 3$ antennas, the proposed test requires $7N-12$ excitations and corresponding measurements per probe location. Using analysis, simulations, and measurements with SAR compliance tools, we establish the accuracy of the method as compared with exhaustive measurements over all possible transmitted signals. The method is useful in evaluating the compliance of portable devices that use codebooks or beamforming as part of their transmission capabilities.

Microwave Emissions From Cell Phones Exceed Safety Limits in Europe and the US When Touching the Body

In our publications, we have shown both from measurements and computer modeling that the specific absorption rate (SAR) reduces by 10%-15% for every millimeter separation of the cell phone on account of rapidly diminishing EM fields in the near-field region of the cell phone antenna. This rapid reduction of SAR depending on the antenna and its location on the handset has been shown, both computationally and experimentally, regardless of the phantom model such as a flat phantom suggested for SAR compliance testing of devices in contact with the body, for a sphere phantom, and for head-shaped models used for SAR compliance testing of cell phones. Unfortunately, our observations in the past were based on SARs of only three cell phones. Expecting that the SARs for cell phones may exceed the safety limits for body contact, cell phone manufacturers have started to recommend that the devices can be used at 5-25 mm from the body even though it is difficult to see how to maintain this distance correctly under mobile conditions. The National Agency ANFR of France recently released the cell phone SAR test data for 450 cell phones that measure 10-g SARs reducing by 10%-30% for each millimeter distal placement from the planar body phantom. Their data corroborate our findings that most cell phones will exceed the safety guidelines when held against the body by factors of 1.6-3.7 times for the European/ICNIRP standard or by factors as high as 11 if 1-g SAR values were to be measured as required by the U.S. FCC.

Near‐field reconstruction for portable wireless devices by deconvolution from input impedance changes

The near-field validation of computational electromagnetic models of personal wireless communication devices is becoming indispensable, especially, when assessing specific absorption rate (SAR) compliance to regulatory limits by computer simulation. A possible practice for such validation is to compare measured and simulated electromagnetic field distributions in a standard flat phantom designed for SAR compliance measurements. The authors recently presented an input impedance measurement-based validation method. They extend their scheme with a deconvolution procedure. This enables near field validation based on the reconstructed E 2 field obtained by the deconvolved measurement of the impedance change compared to the calculated E 2 . However, the reconstruction problem is inherently ill conditioned and requires numerical treatment. They demonstrate that the regularisation of the convolution matrix makes the reconstruction of the E 2 field feasible with promising error. Through examples of a 1 GHz printed inverted F and a 1.8 GHz printed monopole antenna, they show that the mean-square error as low as 1% between the original and the reconstructed E 2 is achievable with the proposed method.

SAR compliance assessment of PMR 446 and FRS walkie‐talkies

The vast amount of studies on radiofrequency dosimetry deal with exposure due to mobile devices and base station antennas for cellular communication systems. This study investigates compliance of walkie-talkies to exposure guidelines established by the International Commission on Non-Ionizing Radiation Protection and the Federal Communications Committee. The generic walkie-talkie consisted of a helical antenna and a ground plane and was derived by reverse engineering of a commercial walkie-talkie. Measured and simulated values of antenna characteristics and electromagnetic near fields of the generic walkie-talkie were within 2% and 8%, respectively. We also validated normalized electromagnetic near fields of the generic walkie-talkie against a commercial device and observed a very good agreement (deviation <6%). We showed that peak localized specific absorption rate (SAR) induced in the oval flat phantom by the generic walkie-talkie is in agreement with four commercial devices if input power of the generic walkie-talkie is rescaled based on magnetic near field. Finally, we found that SAR of commercial devices is within current SAR limits for general public exposure for a worst-case duty cycle of 100%, that is, about 3 times and 6 times lower than the limit on the 1 g SAR (1.6 W/kg) and 10 g SAR (2 W/kg), respectively. But, an effective radiated power as specified by the Private Mobile Radio at 446 MHz (PMR 446) radio standard can cause localized SAR exceeding SAR limits for 1 g of tissue.

Confirmation of quasi-static approximation in SAR evaluation for a wireless power transfer system

The present study discusses the applicability of the magneto-quasi-static approximation to the calculation of the specific absorption rate (SAR) in a cylindrical model for a wireless power transfer system. Resonant coils with different parameters were considered in the 10 MHz band. A two-step quasi-static method that is comprised of the method of moments and the scalar-potential finite-difference methods is applied, which can consider the effects of electric and magnetic fields on the induced SAR separately. From our computational results, the SARs obtained from our quasi-static method are found to be in good agreement with full-wave analysis for different positions of the cylindrical model relative to the wireless power transfer system, confirming the applicability of the quasi-static approximation in the 10 MHz band. The SAR induced by the external electric field is found to be marginal as compared to that induced by the magnetic field. Thus, the dosimetry for the external magnetic field, which may be marginally perturbed by the presence of biological tissue, is confirmed to be essential for SAR compliance in the 10 MHz band or lower. This confirmation also suggests that the current in the coil rather than the transferred power is essential for SAR compliance.

Hand effect on head specific absorption rate (SAR) exposed by two realistic phone models

There have been some reports about possible effect of the hand presence on the head SAR if hand phantom is included in the measurements of the head SAR compliance assessment procedure. The objective of this computational study was to examine the reported effect by using realistic head models and realistic CAD based phone models. A commercially available FDTD based EM solver was used to carry out the computational work. Based on the results of this study considering the SAR values without hand phantom as reference, following conclusions can be made: 1. In general presence of the hand lead to significantly less conservative SAR values in the head for large majority of cases 2. For lower band GSM frequencies the presence of the hand decreases the head SAR up to ~70%. 3. For the upper band GSM frequencies the presence of the hand decreases the head SAR up to ~55%. Based on the results of this study the present SAR compliance protocol where hand phantom is not included leads to more conservative head SAR results compared to the cases where hand is included.

An Algorithm for Predicting the Change in SAR in a Human Phantom Due to Deviations in Its Complex Permittivity

The aim of this study is to determine a robust prediction algorithm that can be used to correct the measured specific absorption rate (SAR) in a homogeneous phantom when its complex permittivity deviates from standardized reference values. Results are analyzed over a frequency range of 30-6000 MHz. Both measurements and numerical simulations are presented. Several antenna sizes and distances to the phantom are investigated so as to study a large range of SAR distributions. It is demonstrated that the prediction algorithm, while developed using dipole antennas, also works well for mobile telephone models. Employing the prediction algorithm reduces the SAR measurement uncertainty, thereby improving the reproducibility of SAR compliance assessment between laboratories. Another benefit of the algorithm is that it enables the use of broadband tissue-equivalent liquids, whose dielectric parameters are not currently within the tight tolerances of existing standards. The use of broadband liquids reduces the cost of SAR measurement. The method presented in this paper is of benefit to the IEEE 1528 and IEC 62209 measurement standards.

New Reference Function for Post-Processing Uncertainty Evaluation in SAR Compliance Tests

A reference function for the post-processing uncertainty evaluation in specific absorption rate (SAR) compliance tests is proposed. The new reference function, derived from real SAR distributions measured during compliance sessions of handsets, is characterised by a double Gaussian distribution with asymmetrical standard deviation parameters, in each half space. Standard deviation parameters as well as SAR penetration are frequency dependent data. The proposed reference function also takes into account SAR distributions with two (or more) local maxima. The analysis for the derivation of the reference function parameters based on GSM900, DCS1800, UMTS and IEEE 802.11b measured SAR distributions is presented as well as details on uncertainty computations. A generalization of the method is presented too.