Nacer Chahat

A Compact UWB Antenna for On-Body Applications

A new compact planar ultrawideband (UWB) antenna designed for on-body communications is presented. The antenna is characterized in free space, on a homogeneous phantom modeling a human arm, and on a realistic high-resolution whole-body voxel model. In all configurations it demonstrates very satisfactory features for on-body propagation. The results are presented in terms of return loss, radiation pattern, efficiency, and E-field distribution. The antenna shows very good performance within the 3-11.2 GHz range, and therefore it might be used successfully for the 3.1-10.6 GHz IR-UWB systems. The simulation results for the return loss and radiation patterns are in good agreement with measurements. Finally, a time-domain analysis over the whole-body voxel model is performed for impulse radio applications, and transmission scenarios with several antennas placed on the body are analyzed and compared.

Millimeter-wave interactions with the human body: state of knowledge and recent advances

The biocompatibility of millimeter-wave devices and systems is an important issue due to the wide number of emerging body-centric wireless applications at millimeter waves. This review article provides the state of knowledge in this field and mainly focuses on recent results and advances related to the different aspects of millimeter-wave interactions with the human body. Electromagnetic, thermal, and biological aspects are considered and analyzed for exposures in the 30-100 GHz range with a particular emphasis on the 60-GHz band. Recently introduced dosimetric techniques and specific instrumentation for bioelectromagnetic laboratory studies are also presented. Finally, future trends are discussed.

On-Body Propagation at 60 GHz

The on-body propagation at 60 GHz is studied analytically, numerically and experimentally using a skin-equivalent phantom. First, to provide analytical-based fundamental models of path gain, the theory of propagating waves near a flat phantom is studied by considering vertical and horizontal elementary dipoles. The analytical models are in excellent agreement with full-wave simulations. For a vertically polarized wave, a minimum power decay exponent of 3.5 is found. Then, propagation on the body is investigated experimentally in vertical and horizontal polarizations using two linearly-polarized open-ended waveguides. The analytical models fit very well with the measurements. Furthermore, the effect of polarization on the antenna performance is studied numerically and experimentally.

Broadband Tissue-Equivalent Phantom for BAN Applications at Millimeter Waves

The extension of body area networks from microwaves to millimeter waves requires to develop experimental phantoms emulating the dielectric properties of human skin for the accurate, reproducible, and well-controlled characterization of wearable antennas, on-body propagation channel, and absorption of the electromagnetic power by the human body. Here we introduce a broadband skin-equivalent semisolid phantom whose composition is optimized to coincide with measured values of the human skin permittivity in the 55-65-GHz range. To confirm the accuracy of this phantom, specific absorption rate measurements are performed at 60 GHz using a temperature-based approach. An excellent agreement between the experimental and numerical results is demonstrated.

Wearable Endfire Textile Antenna for On-Body Communications at 60 GHz

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.

Characterization of the Interactions Between a 60-GHz Antenna and the Human Body in an Off-Body Scenario

Interactions between a 60-GHz microstrip patch antenna array designed for off-body communications and the human body are investigated numerically and experimentally. First, the array is characterized in free space and on a homogeneous skin-equivalent phantom in terms of reflection coefficient, radiation pattern, and antenna efficiency. Second, a multiphysics dosimetry technique is proposed and implemented to determine the specific absorption rate (SAR) and incident power density (IPD) from the heating dynamics measured on an experimental phantom using a high-resolution infrared (IR) camera. The SAR and IPD are found by fitting the analytical solution of the bio-heat transfer equation to the measured heating dynamics. The experimental and numerical results are in a very good agreement. They demonstrate that for the considered scenario the impact of the body on the antenna characteristics is almost negligible, and even relatively high radiated powers (up to 550 mW) result in exposure levels that are below international exposure limits.

60-GHz Textile Antenna Array for Body-Centric Communications

To demonstrate that commercial textiles can be used as antenna substrates at millimeter waves, a microstrip patch antenna array printed on textile is proposed for off-body communications in the 60-GHz band. The textile substrate is characterized in V-band using the open stub technique. A new fabrication process is introduced for the reliable and accurate manufacturing of millimeter-wave microstrip antennas on textiles. The antenna reflection coefficient, radiation patterns and efficiency are studied in free space, with and without bending, and on a homogeneous skin-equivalent phantom. The numerical and experimental results are in a good agreement. To the best of our knowledge, this is the first textile millimeter-wave antenna optimized for off-body communications presented in the literature at millimeter waves.

New Method for Determining Dielectric Properties of Skin and Phantoms at Millimeter Waves Based on Heating Kinetics

Recent progress in millimeter-wave (MMW) wireless body-centric applications triggered an increasing interest to characterize the interactions between the millimeter waves and the human body. The determination of the dielectric properties of skin and phantoms (artificial models with tissue-equivalent dielectric properties) at MMW is crucial for the accurate evaluation of the power absorption and distribution in the skin. In this study, we show that the heating kinetics resulting from the MMW exposure can be used for the accurate determination of the penetration depth (δ) and power density (I) in different samples (1% and 4% agar phantoms, 20% and 25% polyethylene powder (PEP) phantoms, and human skin). The samples have been exposed at 60.4 GHz using an open-ended waveguide. The temperature distribution and dynamics are recorded using an infrared camera. The values of δ and I are defined by fitting the analytical solution of the bio-heat transfer equation to the experimental heating kinetics. The values of δ are further used to retrieve the permittivity spectra of materials described by Debye equation. Simultaneously, δ is calculated using the permittivity directly measured using a slim coaxial probe. Both results are in good agreement. Finally, our results demonstrate that the permittivity of a 20% PEP phantom is close to that of skin. Hence, this phantom can be used to model the MMW interactions with skin and to characterize on-body wearable MMW antennas.

Effect of Textile on the Propagation Along the Body at 60 GHz

The effect of textiles on propagation along the body at 60 GHz is studied. A Greens function representation is used to investigate analytically the field excited by an infinitesimal dipole over a multilayer structure representing a flat skin model and clothing. The propagation between two rectangular open-ended waveguides is studied numerically and analytically in terms of path gain. Furthermore, the effect of an air gap between the textile and phantom is considered. Results show that the presence of a textile over a skin-equivalent phantom, as well as an air gap between them, induces a typical decrease of the path gain by 2-5 dB, but it doesnt significantly affect the path gain exponent. In all considered scenarios the power decay exponent is around 3.6.

Improvement of the on-body performance of a dual-band textile antenna using an EBG structure

A new dual-band wearable textile antenna designed for body-centric communications is introduced. The antenna is fully characterized in free space and on the body model, with and without an electromagnetic band gap (EBG) substrate. The band gap array consists of 3 × 3 elements and is used to reduce the interaction with human tissues. With the EBG back reflector, the radiation into the body is reduced by more than 15 dB. Increases of 5.2 dB and 3 dB gain are noticed at 2.45 GHz and 5.5 GHz, respectively. The results are presented for the return loss, radiation pattern, efficiency, and specific absorption rate (SAR).